Paul Wennberg, Atmospheric Chemist, Environmental Scientist, Instrument Engineer, and Research Leader on the Global Carbon Cycle

How much carbon dioxide is in Earth's atmosphere, and how is the total amount split between naturally occurring and anthropogenic causes? That question is one of the most challenging in all of observational science, and against the backdrop of climate change, it is arguably the most pressing among all the structural problems that humanity faces in the 21st century. In the discussions below, Professor Paul Wennberg recounts his personal journey, and the innovations, collaborations, and breakthrough discoveries that have made him one of the world's foremost research leaders in atmospheric chemistry.

Raised in rural Vermont, Wennberg's love of the forests around his house and the tinkering he could do in the garage would foreshadow his dual interests in environmental stewardship and cutting-edge instrument engineering for science. At Oberlin, he majored in chemistry, and in graduate school at Harvard, his investigations of hydroxyl radical abundances in the stratosphere pulled him into the international effort to study, and eventually mitigate, the ozone hole, which was menacingly growing by the year due to the unconstrained proliferation of chlorofluorocarbons (CFCs) in the atmosphere.

As a young professor at Caltech, Wennberg appreciated immediately the Institute's proud legacy in atmospheric research and a campus culture that was optimized for scientific impact. Following the leading collaborations he had already forged with NASA on aircraft instrumentation, Wennberg steadily became more involved in the study of the sources and sinks of carbon dioxide in Earth's atmosphere. His efforts led directly to the space-based Orbiting Carbon Observatory (OCO) mission, along with its ground-based counterpart, the Total Carbon Column Observing Network TCCON). After a launch failure doomed the original OCO spacecraft in 2009, Wennberg and his collaborators were ready for a re-launch in 2014, and today, both endeavors have profoundly advanced our understanding of the human and natural dynamics of Earth's global carbon cycle.

In all the important recollections and perspectives that Wennberg shares below, readers will especially benefit from his nuanced and deeply felt views on being an environmental scientist and an environmentalist. For Wennberg, there is no inherent conflict between the two; as he has demonstrated, the objectivity that observational science demands is not in and of itself a barrier to the more subjective aspects that propel the politics of environmental activism. As an honored and authoritative voice on the science of climate change, Wennberg has demonstrated the importance of contributing to the enormously complex social, economic, and political considerations surrounding global warming with impeccable and relevant data - and the wisdom to interpret what we should be doing as a result. Toward the end of the interview sessions, which concluded in 2023, Wennberg expressed interest in becoming more closely involved in the policy discussions that ultimately will inform how humanity chooses to confront the challenges that lie ahead. Today, after completing his service as director of Caltech's Linde Center for Global Environmental Science, Wennberg is now a senior contributing scientist at the Environmental Defense Fund. Amid the current political climate, there is no doubt that this work - and the broader effort to continue work in climate science - is now more important than ever.

DAVID ZIERLER: This is David Zierler, Director of the Caltech Heritage Project. It's Friday, February 10th, 2023. I'm delighted to be here with Professor Paul O. Wennberg. Paul, thank you so much for having me today. It's great to be with you.

PAUL WENNBERG: Thank you, David.

ZIERLER: To start, would you please tell me your titles and affiliations here at Caltech? I have your page pulled up, so if you forget one, I can help you out. Let's see what you can do.

WENNBERG: Most importantly, I am a Professor here. I am the R. Stanton Avery Professor of Atmospheric Chemistry and Environmental Science and Engineering, which is actually too long to put into most of the boxes where they ask for your title. I'm also right now the Director of the Linde Center for Global Environmental Science, but my replacement has just been approved by the IACC. I've served in this role, on and off and on again, since the founding of the Center. That has been a major part of my last 15 years here.

ZIERLER: Is it public knowledge who your successor will be, or we don't want to say yet?

WENNBERG: Well, you're not going to tell anyone until this is approved, so—Andrew Thompson, who is a physical oceanographer here, and a great friend, and just an excellent person to take over.

ZIERLER: That's great.

WENNBERG: I'm also currently the Executive Officer for Environmental Science and Engineering, the option or program or department, depending on which division you're talking about. Andrew Thompson will also shortly take over this role.

ZIERLER: Because in EAS, there are departments.

WENNBERG: It's actually a department, right.

ZIERLER: It's not chair of the department, though? It's still executive officer?

WENNBERG: It's called executive officer. That's right. That is effectively the head of the department. That's right. As EO, I serve on the executive committee for Harry Atwater, who is the Chair of EAS.

ZIERLER: I think we've covered it all. Now, between all of that, there's GPS and there's EAS? Are you split evenly between the two divisions?

WENNBERG: Fifty/fifty, that's right. When I came to Caltech, the chair of GPS was Ed Stolper. The chair of E&AS was John Seinfeld. The provost at the time had been involved in trying to understand what Caltech should be doing in the "global environmental science".

ZIERLER: This was Steve Koonin?

WENNBERG: That's right. You can read this as climate and other things. The Environmental Engineering Science (EES) Department, which was the predecessor to ESE, was fully in the Division of Engineering and Applied Science. So, this was dreamed up by Ed, and by John, to jointly put in positions to build something new, effectively, in collaboration with the EES option. After some discussion, we became the ESE option, so we just switched the last two letters — Environmental Science and Engineering. The program was originally housed, as far as the students were concerned, in Engineering. Then when the longtime option manager, Linda Scott, moved to a new position at Caltech, the program effectively got transferred into GPS, simply because of operational questions—who's going to manage the students, and so on.

ZIERLER: The missing link here, though—as an atmospheric chemist—

WENNBERG: Yeah, it's Chemistry. [laughs]

ZIERLER: Yeah, Chemistry! Not CCE.

WENNBERG: Yeah, isn't that funny? So, I am a chemist, and actually half of my research group historically has been students and staff from Chemistry and Chemical Engineering. I do not, however, have a formal appointment in CCE.

ZIERLER: That would just be another division chair to answer to.

WENNBERG: And I don't think you're allowed to do that, actually.

ZIERLER: You max out at two?

WENNBERG: Apparently. It's just fine with me, by the way. I don't need more divisions [laughs].

ZIERLER: When you say fifty/fifty, both in terms of administrative responsibilities, where your students are, where you teach—it's really across the board half and half?

WENNBERG: Effectively, yeah. The Chair of Engineering and Applied Science and the Chair of Geological and Planetary Sciences, I'm responsive equally to both of them.

ZIERLER: Let's go back now to the titles, the people that you're named in honor of. First, R. Stanton Avery.

WENNBERG: R. Stanton Avery.

ZIERLER: Did you meet Avery? Did you ever cross paths with him?

WENNBERG: I did not meet him – he died just before I came to Pasadena. But do you know John Onderdonk is related to the Avery family?

ZIERLER: Is there a specific connection between his interest in chemistry and—?

WENNBERG: Not that I know of. It's a loose tie that maybe you can dig into, but I'm doubtful.

ZIERLER: He made modern stickers, so there must be something there.

WENNBERG: Yeah. Sometimes people, my family would joke that I'm the Post-It Professor.

ZIERLER: [laughs]

WENNBERG: I have to tell them that Post-Its are not an Avery product. [laughs] I forget who that is; 3M or something. [laughs]

The Vision of the Lindes

ZIERLER: I think so, yes. Of course, Ron and Maxine Linde, they've—

WENNBERG: Right, yes.

ZIERLER: Tell me about your relationship with them.

WENNBERG: Have you met them?

ZIERLER: I've met Ron.

WENNBERG: You've met Ron. Which is surprising that you didn't also meet Maxine.

ZIERLER: Well, only over Zoom, unfortunately.

WENNBERG: Oh, okay, okay. Because they—

ZIERLER: They're a team.

WENNBERG: —they are fully a team. Maxine—they told me the funniest story about—Ron did his PhD here. Maxine was trained in mathematics. They had this arrangement, where he would get his PhD first, and then she would get hers second, so they would tag-team. But she worked at JPL, as far as I understand, at that time. Then when he was graduating, he said, "I'd prefer to go to start a business." She thought that was great, and so she went to law school, and then she became head counsel for all of their businesses over the years. They are inseparable. They travel together, and just lovely people.

ZIERLER: Collect art together.

WENNBERG: They do all of that. They have a spectacular house, I've heard, filled with beautiful art. I have not been there to visit them yet.

ZIERLER: Are they involved at all in the Institute? Do you keep them up to date? Do they keep their distance by necessity? How does that work?

WENNBERG: I assume you mean Center? We inform them along the way of what we're doing. I let them know last weekend that I was stepping down and that Andy was going to take my place, and they were thrilled and look forward to meeting with Andy. They're interested in what we do. The environmental sciences I'd say has become more and more of a passion of theirs. I'm not sure it was always the case. When we founded the Center, the environmental scientists and engineers on campus were spread out all over the Institute. Well, our big huge campus, right?

ZIERLER: [laughs]

WENNBERG: Nevertheless, some of us were over here in this corner of the campus. Many were in Keck. Some were in Spaulding. Some in Noyes. There had been this long-standing interest in trying to build a program here and to get people to be co-housed. When the astrophysicists moved across the street, the Robinson Laboratory became empty. It was just this great opportunity. It was fenced in. Were you here on campus at the time?

ZIERLER: No, not then.

WENNBERG: The PMA guys won't like me to say this, but they're well known for letting their buildings decay. [laughs] It's not the prime, primary—

ZIERLER: They're thinking about stars and particles.

WENNBERG: They are. And less about the things of man. And women.

ZIERLER: It's a beautiful renovation in here.

WENNBERG: Yes.

ZIERLER: It's the best combination of a gorgeous exterior, with all of the modernization.

WENNBERG: That's right. So, it was this huge opportunity. It was a derelict building, pretty much, and huge amounts of water damage, downstairs, because of the way it had been engineered. So, there was this opportunity. Ron and Maxine, I think, understood that this was something special, that they could help create something new on campus, and in a historic building. The provost and president at the time, Jean-Lou, and Ed, convinced them that this would be something that they would be very proud of. Now, over the years, since the Center was founded, they became I think more and more interested in actually what humans are doing to the Earth, just like everyone, right? It has become such a center of all of our conversation. I think they also see and have seen their investments as something that they can be proud of, as helping to answer some really fundamental questions in global environmental science.

ZIERLER: With Ron as a materials scientist, and then the founding of Envirodyne, is there a—not that they're closely involved; that's not the role of a benefactor—but is there sort of a founding world view that the Lindes brought to this endeavor, that has—?

WENNBERG: They always said, "We just want the best." That's what you really want, in our benefactors. They're not managing or meddling or anything else. They really just see themselves as champions of excellence, and that's what they aspire to.

ZIERLER: It shows a tremendous amount of trust on their part.

WENNBERG: And trust, yes, in the Institute. Of course, Ron is somebody who has served the Institute, as a member of the Board of Trustees, and then the Finance Committee, and so he has really put—literally put his name on campus, but also just—

ZIERLER: Amazing.

WENNBERG: —just the amount of time that both he and Maxine have invested in supporting the Institute is really exceptional.

ZIERLER: With The Resnick Institute and now sustainability being such a widespread concept and area of research, how has that changed your work as director and what the Linde Institute does? Or has it not?

WENNBERG: RSI, the Resnicks—up until now—has served almost like an internal funding pool for campus. It's more than that, of course. There is a sense of trying to do programming. But I would say mostly it has served as a way for faculty to get their ideas moving. As such, the faculty in the Linde Center have been highly successful in securing financial support from RSI to accomplish what they are interested in. Maybe this will show up in your interview or not, but the Linde Center suffered from an unfortunate coincidence, which was that when Ron and Maxine's very generous gift arrived, it was invested in the endowment, just before the crash in 2008. As a result, program support within the Center itself has been underfunded. We've received a small amount of funds from the original gift, but most of it pays the bond on the building. As a result, then, we've been working to secure most of our support external to that original gift. The RSI effort really helps and enhances the science of the faculty here.

ZIERLER: The Resnicks, who don't have that educational connection to Caltech like the Lindes did, when they decided to support Caltech in this tremendous way, did you personally, or the Linde Institute, serve as sort of a—

WENNBERG: The Linde Center.

ZIERLER: Linde Center. I'm sorry.

WENNBERG: Gotta keep these straight, because the Linde Institute is over there.

ZIERLER: That's over there. Did the Linde Center give Caltech legitimacy in the space of environmental science that convinced the Resnicks?

WENNBERG: I think so. If you go back and you look at the pitch that was made to them, and is in the original—for the second gift, now, we're talking about; the most recent one.

ZIERLER: The big one.

WENNBERG: The big one, 2018. The highlights of the things that Caltech was doing that they could take part in, and that program would leverage, were the ones that were in climate and related sciences, in addition to the work that had been done for Sunlight to Everything, effectively. Those were the two Centers that were shown as Caltech is a place where additional investments would be profoundly useful.

ZIERLER: Between you stepping down as director and the building coming together, for Resnick, will you get more involved with RSI?

WENNBERG: When you started with what are your titles, you didn't mention any associated with RSI, but I am actually very involved. I serve as the Initiative Lead for the Climate Science Program. At some level, I get to wear both hats, and in many ways, they wear the same hat. I suspect I'll just continue in that role, but that's up to Jonas Peters, who is the—I don't know his title, as director. He is the one who has a formal—he'll have a title, I think. All of the rest of us—Mark Simons, Dianne Newman, and Harry Atwater originally, and now Karthish, and me—we were all brought in as sort of like a senior leadership team, but without title.

ZIERLER: Let's go now to the fundamentals in terms of your research agenda. By your academic training, you're an atmospheric chemist.

WENNBERG: That's right.

ZIERLER: But you built a lot of stuff, too. Do you also consider yourself an engineer?

WENNBERG: Yes.

ZIERLER: You're both.

WENNBERG: I would certainly call myself an applied scientist. I don't have formal engineering training, but we build a lot of stuff, and I've been heavily involved in instrument assembly and design. So, although I don't have the formal engineering background, it's something that I really enjoy, and have enjoyed a lot while I've been here at Caltech.

The Scientist as Instrument Builder

ZIERLER: Now, did you tinker around when you were a kid?

WENNBERG: Oh, yeah. Yeah. I grew up on a small farm, and tinkering was what you had to do.

ZIERLER: For most scientists engaged in fundamental research, they outsource the instrument-building.

WENNBERG: The tinkering.

ZIERLER: But you never did?

WENNBERG: No. See, I'm still playin' around with parts, here—

ZIERLER: [laughs] What has been advantageous and challenging, to wear both hats?

WENNBERG: You mean design and also science?

ZIERLER: Yes.

WENNBERG: I think it's a continuum. You build machines because you need a machine to answer a question that you're interested in. And also, because it's really fun. It's fun to create something. I find it very enjoyable, and I like to see machines that we build do things that no one else has been able to do, to be able to measure things no one has been able to measure.

ZIERLER: Of all the machines you've built, what are the connecting threads? What are the similar kinds of questions and objectives?

WENNBERG: "What's in the atmosphere, and why, and what's happening?" Those are the key—"What's the composition of Earth's atmosphere and then how does it evolve?" My original scientific effort was at Harvard in my PhD, where I really got into this, and it was building instruments to make measurements in the stratosphere to understand stratospheric ozone chemistry. Then when I came here, I began a program in more tropospheric chemistry—so that's the chemistry occurring where we breathe, everything from background chemistry of the natural environment to the polluted chemistry of Los Angeles. I also had a number of friends up at the Lab, at JPL. Pretty soon after I got here, a small group of us were talking about—climate was clearly going to become, and was already, the sort of generational question in environmental science. What could we do? What could we build to help answer some of the key questions? A small group of us—there were only maybe five or six at the time—Ross Salawitch, who is now at Maryland; Bhaswar Sen; Geoff Toon; Chip Miller; myself; then later, David Crisp, and Professor Yuk Yung—dreamt up this idea: could we measure—could we make global maps of carbon dioxide from space, using optical spectroscopy? My PhD thesis used optical spectroscopy for measuring free radicals, so this wasn't a terrible stretch, although the remote sensing stuff, I had never done before. We had to convince ourselves we could do that, so in my group we built one of the first spectrometers to show that you could do this from the ground. What we're measuring is not the amount of CO₂ in a small amount of air —there's a long history such measurements in ambient air. In fact, it was sort of invented here at Caltech by David Keeling, in the geochemistry labs. But here what we're measuring is not the amount of CO₂ in a cubic centimeter of air, but rather the total column of CO₂. That's the amount of CO₂ between the surface and the top of the atmosphere.

ZIERLER: By column, you mean planet-wide?

WENNBERG: Yes.

ZIERLER: Not just a sample.

WENNBERG: No, so you're measuring over every meter, or hopefully over almost every meter, the amount of CO₂. You also measure the amount of oxygen. That's the way you do this experiment. Then by dividing those two, because oxygen tells us how much dry air there is that's in that column, which varies because of surface elevation changes, or because of scattering of particles, then by dividing the CO₂ and O₂ columns, you can get the mixing ratio, the same units that we measure in in-situ measurements, of parts per million. You can get the mean mixing ratio in the whole column. The cool thing about this now is that you get global maps of the mass of CO₂ over the Earth. With that data, now you can see there's a little bit more here than there is over here. So, we can, let's say, invert this observation to ask, how big of a sink, how much CO₂ has to be pulled out as the winds blow from here to here, in order to produce the patterns of CO₂ that you see? Or maybe there's more CO₂. Like if we take the data over Los Angeles, it has more CO₂ than the air outside of Los Angeles, because of all of our emissions. We can use this data, then, to estimate how much CO₂ is being emitted in the cities of the world. While that number is something that is known pretty well for Los Angeles, it's not nearly known well enough for lots of other cities in the world, because of challenges of reporting, or potentially underreporting or overreporting because there was some economic incentive to do that.

ZIERLER: Is extrapolation valuable, just roughly the size of Los Angeles, that you can say basically we would expect the same from similar size cities?

WENNBERG: No, it's very much related to the intensity of the industry in those cities. It's not just population density. Per capita, CO₂ emissions in New York City, for example, are much, much lower than they are in Los Angeles. There is actually a pretty strong relationship between population density and CO₂ emissions per capita, and it goes down. As the density goes up, then the CO₂ emissions per person go down. You can think of all the reasons why that might be—more efficient, from an energy point of view, to heat a building, than to heat thousands of houses, just the numbers of meters squared of wall, per person. Then of course transportation. So, yes, we're learning a lot from this. The OCO—we called this mission OCO, which is sort of a bit of a joke, because OCO is the chemical formula of CO₂, so it's symmetric with O, carbon, O, oxygen, carbon, oxygen. But it's called the Orbiting Carbon Observatory, and we launched in 2008, and it went into the drink. The rocket failed to—well, the fairing failed to separate from the payload, and so this instrument that so many people had worked on failed to make it to orbit.

ZIERLER: What actually happened?

WENNBERG: The fairing, which is a cover—you have the rocket below you, then you have the spacecraft, and on the spacecraft you have your instrument. To protect the instrument and the spacecraft during launch, they put what's called a fairing on top. Think of it as just like a plastic shell, a hard plastic shell. Once you get up through most of the weather and the atmosphere, there's a squib that fires that releases this, these shells, so that the payload now is free-flying. But with this failing to separate, the fairing, now the whole thing weighs too much. Plus, you would never be able to see any light, because you've got fairing out in front of you. So it's basically a suborbital trajectory into the Southern Ocean.

ZIERLER: No appetite for relaunch?

WENNBERG: No, there was!

ZIERLER: Oh, there was.

WENNBERG: 2008. Recession. Unfortunately for the Linde investment in the Center, but you remember there was a call for ‘shovel-ready' projects, things that the U.S. government could spend money on to stimulate the economy. And we put up our hand – here Ross Salawitch led the charge - and we got selected to build another one. Five years later, we launched, and the mission is going very well. It's called OCO-2, but it's really OCO.

ZIERLER: Improved technology?

WENNBERG: Nope. It was 100% rebuild, including several design flaws that we didn't appreciate until the successful launch.

ZIERLER: I guess that's shovel ready.

WENNBERG: Shovel ready. In fact, though, for OCO-2 they decided to build a spare instrument, and so they built a second instrument, just in case, I guess. Once OCO-2 was successfully in orbit, the question of what to do with the spare instrument was of interest to a lot of people. Now, that's up on the space station, OCO-3. But it's fundamentally just a spare of the original with added transfer optics to get the light into the instrument and the payload interfaces to the station.

ZIERLER: It's not just Earth for you; you're involved in Martian climate research as well?

WENNBERG: No, no, no.

ZIERLER: That's just a sidebar.

WENNBERG: Actually, it was more than a sidebar. It was yet another example of the risks of being involved in space missions. Over the years, I had been involved with conversations about—because I'm here with all the planetary scientists—about planetary atmospheres. The way our instruments—there are ground-based instruments for doing CO₂, of which one is in this building, actually. We take spectra of the Sun in the near infrared, and what you see then is the strong absorption features, the lines of absorptions, from all the greenhouses gases. By measuring and retrieving from those spectra how many molecules of CO₂ et cetera are between the ground and the Sun, we basically get from the ground what we're trying to measure from space. This method of using a Fourier transform spectrometer to take spectra of the Sun and get basically the limb transmission of—or the atmospheric transmission of a very bright and fairly parallel light source.

ZIERLER: What does that mean, parallel light source?

WENNBERG: You think about the Sun as not very big in the sky. It's a very bright, intense object. The rays are coming directly at us. If you just take spectra of the middle of the Sun, all you're looking at is a direct transmission spectrum of the sunlight. That's different than if you look at the sky. Here, you can imagine the Sun is over there, it's scattering the light off of the sky, and then coming down to you, and you might measure the spectrum of the sky. But that's a very complicated retrieval question, because now, you need to understand the scattering environment and so forth. With the solar spectroscopy, there's none of that. It's simply a geometric problem. You know the path length directly from the Sun to the spectrometer.

ZIERLER: How does this show us carbon levels?

WENNBERG: For us, we get exactly the same thing you get hope to get from space, which is we get the number of CO₂ molecules between the Sun and the ground. We get the number of oxygen molecules between the Sun and the ground. Or methane, or other gases. We divide, and we get the mean mixing ratio along that path. It's just very precise and accurate.

ZIERLER: Of course there's lots of orbiters that are doing the same kinds of measurement. Do you use it as a check, or—?

WENNBERG: We use it as a check. We built out a whole network of these instruments. There's about 24 of them around the world now. I led that network for a long time, probably 15 years, as it got built up, and now it has been passed on to others, the young folks who are doing great. Debra Wunch, who helped develop the network as a postdoc and later as a Caltech staff member is now the Chair of the network which she leads from her position as a faculty member in the Physics Department of the University of Toronto. There's this whole network of these instruments around the world, run by lots of individuals, to produce this amazing dataset that now is used by many, many different remote sensing programs from space as their primary validation data set. We've also been able to use the data for doing a lot of carbon cycle science, just using these data alone. Because we measure lots of things that they don't measure.

Origin Story of Interest in Global Warming

ZIERLER: Just a generational question—when you got involved in measuring carbon, all the way back in graduate school, was the correlation between emissions and warming established at that point, or you were part of that generation?

WENNBERG: In my thesis work, I was actually working on something entirely different. I was working on why was stratospheric ozone going away, and how was that related to the chlorofluorocarbon emissions. It was really a quite different topic. But Al Gore came to Harvard in, I think, 1985 and gave a talk there about the global warming. This had been a major topic of interest in the scientific community for decades. It wasn't like this was some surprise or anything. This was very well known that this ‘experiment' humans were doing was likely to be profoundly important. I think in the 1980s and the early 1990s, what happened that was really quite prominent was that the signal of climate change really appeared out of the noise. There's a lot of variability in the climate system, but by the time we get into the 1980s, it's pretty clear that the model calculations for where we were going were not out to lunch, and that there was significant peril. Even while I was working on stratospheric chemistry, I was turning an eye towards trying to think about the climate problem. Interestingly, the problem I did work on, which was stratospheric ozone chemistry, what we learned was that emitting chlorofluorocarbons, CFCs, was a tremendously bad idea, that these chemicals, which live 100 years in the atmosphere just are very, very potent at removing ozone in the stratosphere. Not the chemicals themselves, but the subsequent chemistry. As a result of the work of the broader scientific community, there was a long diplomatic program that was established, first to study this, then to answer what should we do with it, and then to actually affect the change.

ZIERLER: This was the Montreal Protocol?

WENNBERG: It became the Montreal Protocol, yes, and the United States was a leader in all of this, and NASA was a leader, and NOAA. The interesting thing is that the Montreal Protocol was probably the most effective climate treaty ever to have happened. Because the chlorofluorocarbons are just incredibly good greenhouse gases. And so, if we had continued—

ZIERLER: It's a double win.

WENNBERG: —it is—if we had continued on the path we were going with the CFC production, today they would rival CO₂ as the most important greenhouse gas.

ZIERLER: Wow.

WENNBERG: You know how you go, CO₂, and then you go methane, and—yeah, CFCs would have been right up there with number one. Yeah, we were really lucky.

ZIERLER: You called yourself an applied scientist. The motivation to understand the connection between CFCs and ozone depletion, even in graduate school did you have that sort of crossover fundamental science because you wanted to do something with applications?

WENNBERG: Yes. I was an environmental scientist. I wasn't just a chemist.

ZIERLER: And environment science where—almost also an environmentalist.

WENNBERG: Yes, and an environmentalist. Profoundly interested in learning things that would help us understand the Earth and how to protect it.

ZIERLER: How do we complicate the success story of reducing CFC use and the ozone being fixed? Because it's obviously a more complex narrative than that.

WENNBERG: Yes, it is. Because these compounds live 100 years, we've seen the ozone levels starting to go up again a little bit, but they're still far below what they were, say, in the early 1980s, in terms of stratospheric ozone. Because it just now is just a huge slog to wait for these chemicals to be cleaned out of the air. I think one of the things it really gives you appreciation for is how careful you need to be in releasing large amounts of long-lived chemicals. We really didn't understand just how impactful these CFCs would be, really until the ozone hole appeared. Fortunately, that was the canary. It was a motivator for action in a way that I think—you know the frog analogy of—? Yeah. That's one of the challenges with the broader climate problem. We're putting crap tons of CO₂ in the air. It has a lifetime that—well, many different lifetimes. But we're doing a 100-year-lifetime experiment here, where we don't fully know the answer, and yet we're dumping stuff that's going to last a long, long time.

ZIERLER: When you say many lifetimes, what does that mean?

WENNBERG: CO₂ is tricky because on one hand, it has a first-order short lifetime. So, about half of every ton of CO₂ we put in the air gets removed in the following few years. So, half of it goes away, into the oceans, and into the biosphere. So there's this one short lifetime where the amounts of CO₂ in the surface oceans and in the above and below ground biomass quickly respond to that. But then, you have a much, much longer timescale, which has to do with the CO₂ making its way into the interior of the ocean. This can be a hundred to thousands of years. Then the eventual burial into calcium carbonate rocks is something that takes thousands of years. That's what I meant by many lifetimes. There's a short response, there's a medium—well, medium being hundreds of years—and then there's this centuries-long lifetime that has to do with its inevitable burial.

ZIERLER: Do you call yourself also a climate scientist?

WENNBERG: Yeah.

ZIERLER: It's such a broad field. What do you see—?

WENNBERG: My science today—partly because I grew up in Vermont, I'm really interested in the forests, and how they are evolving in response to climate, and to CO₂. Because CO₂ is plant food. So, there's a response of forests to elevated CO₂ which involves two things, one of which is there's more CO₂, so they can inhale more CO₂ while releasing less water. That, they like a lot. The other part, though, is that as the climate warms, this changes the forests, because different types of trees grow differently at different temperatures. I've been working on the northern forests, in particular. These are the forests that exist between Vermont and the Arctic Ocean. It turns out that's where a large fraction of the carbon in the near-Earth system is held. The soils, the below and above-ground biomass there is really high. You think about the Tropics as being this area of very verdant and lots of carbon on the land. It's true—the above-ground biomass there is pretty big—but the below-ground biomass is not so big.

ZIERLER: Why is that?

WENNBERG: It's warm, so the microbes are very happy, and they just respire the tree litter quickly. In the high latitudes, it's cold enough that you can, every year, pile more carbon onto the land. So, today, I mentioned that about half the carbon goes away every year that we put into the air. It's about equal between the land and the ocean. But most of the carbon that's going into the land is going into those forests, these high-latitude forests. We're trying to understand—these forests are providing this huge ecological service for us right now and taking up a lot of carbon. What are the dynamics of that? How is that changing? Why are they growing faster? Is it the CO₂? Is it the temperature? Those are the kind of questions we're trying to answer.

The Imperative to Stop the Burning

ZIERLER: Do you promote the idea of tree-planting on a global scale as a mitigating strategy?

WENNBERG: I think we need lots of different strategies. There are places where afforestation, where we regrow trees, is helpful. But it's overly sold as a solution, in the sense that I think there—this is actually probably true of almost all climate solutions that you hear about—they tend to have salespeople who tend to overestimate their potential. This is why I really feel like you need many, many different solutions to both remove CO₂ from the air and, especially, just stop emitting so much. We've got to stop burning stuff.

ZIERLER: In your decades of research, what are some of the top trendlines that are most obvious to you? What has happened over the past decades?

WENNBERG: On the air chemistry part of what I do, there has been this fascinating change in air pollution. There's this overarching trajectory that we've seen. The United States, for example, through the 1950s and 1960s, had worsening and worsening air quality, until it got bad enough that we did something about it. Caltech played a big, big role in this. Caltech played this important role in first explaining why we had bad air quality, helping to determine how you would mitigate it, and then promoting those policies that led to the Clean Air Act and other really important policies that have led us to have much, much better air quality. That trajectory has been followed across the world. Europe was much later than us in addressing their air quality problems, partly because of the Eastern Europe, Central Europe, Western Europe challenges. Southeast Asia was really late. Huge burst of industrial activities for the last 20 years. But even in China, air quality is now improving really fast. You don't hear that story very much. But following the Beijing Olympics and other things, it was very clear that even with governments that are less responsive to their citizenry, people won't tolerate poisoned air, when they're wealthy. This is I think one of the biggest trajectories, as you were saying, about air pollution chemistry, is this generally cleaning of the air now, across the world, including Southeast Asia.

ZIERLER: What accounts for it? What have been the big human innovations that have allowed for these positive developments?

WENNBERG: Catalytic converters for cars, and just better controls on industrial policies. We did an experiment in 2016 called KORUS-AQ—it was a NASA airborne experiment—to look at the air quality challenges of Seoul, Korea. At the time, the Koreans, their general commentary that you would hear in the news there, and you would hear when you talked to people, is, "Blame China. We're downstream from China, and we have terrible air quality in Seoul, but it's this import of pollution from China. There's nothing we can do about it." We went there, and together with Korean scientists, made a whole host of air chemistry measurements, and showed that a significant fraction of their air pollution problem was local. It was due to the solvents that were being used in Korean factories. Particularly some of these aromatics like toluene and other chemicals were just hugely enhanced in their cities. And I think it did lead to—now you have this group of international scientists who are preparing reports that the government is interested in, and they know what to do. It doesn't take new invention; it takes the application of existing control technologies to their factories.

ZIERLER: Do you see Los Angeles as an exportable success story?

WENNBERG: Yes. And it's not done yet. We still have our own air pollution challenges. But yes, I do think L.A. is that example, of you can have a vibrant, economically vigorous city, with lots of people in it, and, at least sometimes, good air quality.

ZIERLER: As EVs become more and more widely adopted, what will that do?

WENNBERG: Almost nothing.

ZIERLER: Really!

WENNBERG: Yeah. Almost nothing.

ZIERLER: So all of these millions of cars with gas-powered engines—

WENNBERG: Isn't that just incredible? Do you know that the lawnmowers that we use, and leaf blowers, contribute more than all the cars?

ZIERLER: I'm in South Pasadena, and they banned leaf blowers. We're the first city in the country to do it. It's gonna happen—

WENNBERG: It's happening, yeah. It was funny, I was walking my dog two or three days ago, and I saw the small businessman gardener working on a neighbor's house, and he had one of the new battery-operated leaf blowers. I just chatted to him and said, "What do you think?" He said, "Oh, these things are so great. I don't go home smelling like gasoline anymore. They're so much less noisy. They're lighter." So, it's coming. Those electric leaf and lawn equipment, that's going to make perhaps a bigger difference than switching to EVs.

ZIERLER: So why are cars not making a big difference?

WENNBERG: Because catalytic converters are just so amazingly good. They don't have them on those leaf blowers.

ZIERLER: To be clear, we're talking about air pollution. We're not talking about carbon emissions.

WENNBERG: We're not.

ZIERLER: That's a totally different camp.

WENNBERG: It's a totally different camp, yeah. Yes, transportation in California is currently responsible for about half of our greenhouse gas emissions. Power generation, electricity, is about 15%. The rest is agriculture and heating. So, if we could transition the car and truck fleet to electric propulsion, we could knock out half. Even with today's current electricity mix in terms of natural gas and so on, it would have a huge climate benefit in terms of the amount of CO₂ being emitted, because the cars are just so much more efficient, and you get much, much less CO₂ from the electricity that you use to drive your car than from fuel.

The Big Methane Mystery

ZIERLER: To go back to my question about those top trendlines, you first answered on air pollution. What about on carbon emissions? What are you seeing there?

WENNBERG: This is not work I'm doing myself, but I like to be hopeful here. I feel like the economics are lining up now to do the right thing to stop burning stuff. The evidence in the atmosphere is pretty weak. In other words, CO₂ continues to go up at a pace that's unmeasurably different from what it was last year. Methane is accelerating.

ZIERLER: Why is methane accelerating?

WENNBERG: We don't know. That's something I'm very curious about. It shouldn't be.

ZIERLER: Because policy would suggest that there should be restrictions or at least a plateau?

WENNBERG: The U.S. says, for example, that our methane emissions are going down.

ZIERLER: Is it possible there's natural geophysical processes at play?

WENNBERG: The isotopic evidence from methane is that it's getting lighter, which tends to point towards more biological sources. But why that's happening—there's not a lot more cattle. There's not a lot more rice production. And so, it's not at all obvious why methane is going up so fast right now. It's a real mystery and one we need to solve.

ZIERLER: Industrial leaks wouldn't explain it either?

WENNBERG: Those tend to be isotopically very, very different. Those that are coming from, say, natural gas, or fossil gas has a distinctly different—and it is a big focus right now. I'm involved as an advisor to—the Environmental Defense Fund has a project called MethaneSAT, which is launching a spacecraft next year to map, at high resolution, oil and gas facilities, to look for fugitive leaks, as they call them, the ones you're talking about here.

ZIERLER: Is this a JPL project?

WENNBERG: It's philanthropy. Completely built by a private—wholly owned by EDF, the Environmental Defense Fund.

ZIERLER: What's the technology? How's it going to find that?

WENNBERG: It's very much like OCO-2. It's a spectrometer that looks at reflected sunlight, but it looks specifically for the methane.

ZIERLER: That will source it, like where it's coming from?

WENNBERG: It will be able to pinpoint individual facilities that are leaking. It's pretty cool.

ZIERLER: This must be very worrisome, that methane rates are increasing.

WENNBERG: Yes. If we can staunch some of the obvious links from the oil and gas system, if we can figure out why the biological methane is going up so fast, we really need to turn this around, because it's definitely not going in the right direction.

ZIERLER: This is almost like the new CFC story.

WENNBERG: It is, although I don't want to over—again, I don't want to put the thumb too much on it, because it is important, but CO₂ is still the dominant greenhouse gas, and likely to be for the next haul. But what I was going to say was that I think—

ZIERLER: I meant in the sense that this is—the challenge with carbon is it's systemic.

WENNBERG: It is.

ZIERLER: It's the thing we use to live.

WENNBERG: Yes.

ZIERLER: CFCs, we can change the chemistry. Methane, we can find what it is. I meant in that sense.

WENNBERG: Yes, that's true. But methane has a decade lifetime. What's nice about that is it would seem to be much more amenable—if you could figure out how to staunch the sources, you would see the response in the radiative forcing from atmospheric methane fast, because the perturbation only lasts a decade. But what we're seeing right now is it's just going up really fast. It's not good. Not good. But why I'm an optimist, methane aside, is simply because things that were not possible, that you had a hard time seeing how we were going to transition the world's energy supply to low carbon, now seems imminently doable, from an economic point of view. It's no longer a question of how much are you willing to invest to do the right thing. It's how much are you going to save when you do the right thing.

ZIERLER: It sidesteps the whole moral and political will issue.

WENNBERG: That's right. EVs, not all of them but most of them, are just incredibly nicer to drive. Now imagine it costs the same as buying a—

ZIERLER: And we're getting there now.

WENNBERG: —ICE engine. Yep. So, why would you buy that? I've been an optimist for the last decade, I'd say.

ZIERLER: Why would the gardener buy the gas leaf blower?

WENNBERG: "Why? Why would you do that? It makes no sense anymore." That's the way it's going to be with EVs in another few years. People are going to go, like, "Why would I do that? Why would I buy a gas-powered car?"

The Policy Side of Carbon Emissions

ZIERLER: On the climate side, these obviously have policy ramifications. Are you involved on a consulting basis, on an official basis, with the U.S. government, with the IPCC? What kind of affiliations do you have in that regard?

WENNBERG: I do act as—like many scientists who are doing climate science—on advising different groups, including IPCC. The U.S. government as well. One of the interesting things—I had the opportunity to go to the CIA, because—and I didn't even know this—if there are international treaties, it is the CIA that's responsible for monitoring compliance. Who knew? I didn't realize. In this case, they wanted to know what technologies were available to assess whether countries were complying with, say, a treaty to limit carbon emissions. So, that was kind of fun, just to go talk to them. I said, "Can't you just count the coal trucks that are [laughs]—might be easier than doing it through CO₂." But, it's good to have a top-down view on carbon emissions, for sure.

ZIERLER: What are your feelings about the IPCC?

WENNBERG: The IPCC grew out of the same process that led to the Montreal Protocol. I think it is underappreciated how much not just the processes but the players were the same. Because it had been such an effective organizing way to do both assessment and then provide some answers to the economics, it served as an important data point for governments. Because the IPCC—the WMO, the World Meteorological Organization, that organized the ozone assessment, those assessments were highly technical and data-based. It was really a consensus document from a group of scientists and engineers, about both the science and then the technical feasibility of various policy choices. But it was not a policymaking body. In the same way, the IPCC serves that function. It's a way to organize the scientific community and the engineering communities around the climate science. It has its strengths and weaknesses. The conclusions often get watered down because you're trying to reach a consensus. Most people think of this as being a—if you ever watch 20 scientists in a room trying to come to consensus, they quickly whack down anything that is at all a stretch or not fully supported – scientists tend to be a conservative bunch. The IPCC tends to be that same way.

From the perspective of the community, although I'm not involved in climate modeling, the IPCC has become somewhat of a gorilla around the necks of lots of the climate model centers, because there's this paramount need to do assessments every few years. These assessments are hugely expensive. You have all of these centers who are trying to run their climate models and adapt them for the newest thing. It tends to become—I guess you could call it an albatross, in that way. You're on a train that you can't get off, and the ability to do innovation, for example, is stymied by the fact that you just have to keep the train running. That's why I'm so excited about the CliMA effort here on campus that Tapio Schneider is leading together with oceanographers from MIT. Because unlike a lot of the climate centers, they don't have this—

ZIERLER: Institutional baggage.

WENNBERG: —institutional baggage that needs to be fed, so they're able to really go out on those limbs, and build new things, and try new ideas out, and be really fresh about how we're thinking about climate modeling.

ZIERLER: How have you dealt with skepticism within the scientific community? Forget the deniers and the Republican Party and that kind of thing, but the Will Happers and the Richard Lindzens—

WENNBERG: [laughs]

ZIERLER: —and the Steve Koonins of the world. Do you engage in any of that?

WENNBERG: No. I really don't, to be honest.

ZIERLER: Because it's not your personality? Because it's not scientifically interesting?

WENNBERG: Probably be impolitic to say, but I think there's a part of that whole conversation which is very much ego driven. People who want to be important, and this is an avenue to have a soapbox. I don't want to be part of that. When you point out to some of those folks that their slide decks are really old and that there's much better things they could read, and they're not curious about that, and they continue to show their old slide decks, it—

ZIERLER: What is lacking in those old slide decks?

WENNBERG: They often are based on a few papers that are wrong, and have been shown to be wrong, but yet they keep showing up. Whack-a-mole. At some point you start to think that this is not a serious criticism. It's really important for the scientific community to be very critical of everything we do. In fact, most of us are our own biggest critics. So I'm not trying to say that there isn't valuable critical work that needs to be done, and we all need to hold strong criticism not at bay, but you want it to be honest, and you want it to be based on profound knowledge of the field. The folks you listed, with the exception of Dick Lindzen, are not disciplinary scientists. They came at it with some feeling – today I guess the word is ‘vibes'. They often tend to be physicists. There's something about the physics people.

ZIERLER: They're very smart, the physicists.

WENNBERG: They know they are.

ZIERLER: [laughs]

WENNBERG: Yeah. I don't know if that's helpful. I tend not to engage.

The Known Unknowns of Climate Change

ZIERLER: What about in your own mind? Where is there still doubt about just correlating human activity versus cyclical processes?

WENNBERG: Oh, cyclical processes?

ZIERLER: Meaning we know the Earth is warming. Nobody is arguing about that. But then the obvious question is, to what extent is it human activity that's exclusively causing it?

WENNBERG: Well, okay. I think there are important feedbacks in the system. The climate system is very complicated. We have been learning a lot about those feedbacks, so there is internal variability in the climate system that gets amplified by what we're doing. I see in your question this idea of, can we say it's 80% this, and 20% that, kind of an answer, and I'm not sure that's actually a useful way to think about the climate problem. Because as I mentioned, let's say we dump a bunch of CO₂ in the air, and the forests warm, the forests suck up a bunch of CO₂, would you say that's a natural thing or not?

ZIERLER: Right. That's just semantics at that point.

WENNBERG: It is semantics. The question is, what is the forcing doing to Earth? This is not a terribly complicated question. We know that we have substantially increased the rate of forcing by greenhouse gases in the Earth, and that you would be unbelievably surprised if that didn't lead to warming. In other words, the null hypothesis here is not whether it would warm or not, but whether it would warm by three degrees if you double CO₂. That's the null hypothesis. Then you could say, "Maybe there's some really interesting things that make it warm by only one and a half. Or maybe there are other interesting things that lead it to warm by four and a half, because of feedbacks." Those are the types of questions that are still incredibly important and not fully known. We don't really have a good answer for even what the equilibrium climate response is to doubling of CO₂. We know that the basic thermodynamic answer is going to be around three degrees, but with what confidence do we know it's not four and a half, or one and a half? Because those two different answers are really impactful. If it only warms by one—only—warms by one and a half degrees, your policy response would be really different than if it warmed by three, and likewise if it's four and a half. Those are really different Earths. That's really where the questions I think remain, is not this, "Is it 80/20?" but how big are the feedbacks in the climate system, through the clouds, through the biogeochemistry, the biology on Earth, and how do those then either amplify or diminish the net response of the whole system.

ZIERLER: Do you see your research specifically addressing mitigation strategies?

WENNBERG: Methane, I've been working on a bit. In that sense, by probing and trying to understand the sources today, where—for example, the work I'm doing with MethaneSAT as an advisor—that's truly mitigation work. It's trying to figure out how can you organize the observational program to identify leaks in the oil and gas system and then patch them. We've done work, for example, on Los Angeles, looking at the methane emissions in the L.A. Basin. We can show, for example, there's a lot of methane being emitted in L.A., and it's coming from the natural gas infrastructure. In that regard, we've been trying to work on understanding where those leaks actually are happening and how do we—again, once you know where they are, you can hopefully do something about them.

ZIERLER: What about on a planetary scale? If you're not specifically involved, what are your thoughts on geoengineering, sequestration, all of the above? As an environmentalist, perhaps. How are we going to get ourselves out of this?

WENNBERG: I was on the National Academy committee that recently released a report on geoengineering by reflecting sunlight. We advocated for a substantial U.S. investment in research around increasing the reflectivity of clouds, or in the stratosphere aerosol, as a way of understanding how those tools might work, both what the risks of them are and what the potential for ameliorating climate change. It's a time constant problem. You remember we mentioned this 100-year problem. I could see a scenario in which we decide globally—the big "we"—that we're going to prevent the temperature of Earth from going up anymore, while we work on pulling CO₂ out of the air and reducing the amount, so that we don't have to do this forever. Because it is a forever problem if you don't mitigate CO₂ emissions.

ZIERLER: Because it's locked in.

WENNBERG: Yes, but also because absent stopping the emissions we would have to maintain and increase the engineering forever. So, I could imagine an agreement to enhance the sulfate, for example, of the stratosphere, reflecting sunlight, making the Earth not warm anymore, in the context of an agreement that also included investments in carbon dioxide removal - CDR. Because you would have to pair these two, unless—there's no moral—

ZIERLER: Otherwise you have planes flying around forever.

WENNBERG: Forever. Forever. And if they stop flying around, you're committing yourself to the whole of global warming in a few years. So, it's not like you slow it for a little while, and then you go back to the pace you were warming. It's like you snap back to the trajectory you were on. Yeah, it's profoundly dangerous. So the only way you could do this, I think in a moral way, would be to couple the two. Then you asked about carbon dioxide removal. Not playing in that arena right now. As part of investments by the RSI climate program, we have funded a lot of projects on campus, in CDR. Jess Adkins had one that was going before RSI got involved, funded by philanthropy, and he started a company. Harry Atwater started a company. Some of Michael Hoffmann's students, postdocs, staff have started a company. This is a space that's getting to be exciting right now, in part because of the potentials to pay for it. Because it has always been the problem who would pay for CDR, and now we have a partial answer, which is the US government will pay. The question will be, can you spin up the learning enough to make it cheap enough that it's useful.

The Challenge of Sequestration

ZIERLER: Are there different approaches to sequestration, different ways of pulling carbon out of the air? Is there one that strikes you as more feasible than the others?

WENNBERG: You can remove carbon dioxide from streams that have high amounts of CO₂ in it from a combustor. This could be a combustor that's combusting biomass, or fossil fuels. The higher the concentration of CO₂ in the stream, the more easy it is to pull it out. There are companies that are working on trying to extract it directly from air. That turns out right now to be pretty expensive. Extracting CO₂ out from say a smokestack is not nearly as expensive. But then you have to bury it. There's geological storage, is one way. There are a few places in the Earth where you pump the CO₂ down, and it reacts with the rocks to effectively become mineralized. That seems to have a pretty good potential for the kind of storage you want to do, where it doesn't come back out at you again. Then Jess Adkins' work is on trying to add it to the ocean by mixing carbon dioxide with limestone. That's the way the Earth will eventually get rid of this carbon pollution we've put up.

ZIERLER: Long after we're gone, you mean.

WENNBERG: Yeah. That is the long-term answer, that you take limestone, and you mix it with atmospheric CO₂, and you make bicarbonate, and that's in the oceans. Most of the carbon in the oceans is bicarbonate. What Jess and a few other different groups are trying to do is figure out how you can make that process go a lot faster. That also seems pretty promising to me.

ZIERLER: Does carbon needs to be a waste product? Why must we bury it? Can we use it?

WENNBERG: [laughs] Yeah. There's the groups that are thinking about, "Capture CO₂ and use it."

ZIERLER: I mean, we covet our carbon fiber bikes, right?

WENNBERG: Yeah. Just to put this in perspective, the average family in the U.S. emits the weight of their house every year in carbon dioxide.

ZIERLER: It's more than we can use. [laughs]

WENNBERG: [laughs] You could build yourself a new house every year out of these wonderful carbon products that came from the CO₂ but pretty soon you're going to run out of land to put all this on.

ZIERLER: It's way too much, basically.

WENNBERG: It's fantastically—the amounts are just almost unimaginable, how much CO₂ we're putting into the air. [laughs] If you said, "I'm going to make it into some product," you quickly run up to the, "Who the hell could use all that carbon?" [laughs]

ZIERLER: Maybe this is an even more outlandish question—not burying it, but going the other way, bringing it to space.

WENNBERG: Oh, yeah, no.

ZIERLER: Just impossible. What's the science 101 behind that, and why is that impossible?

WENNBERG: Just calculate the tons of CO₂ we're emitting, and calculate how expensive it would be to get it off of Earth's gravity.

ZIERLER: Because the only way to do it is a space launch.

WENNBERG: You have this basic problem, that you're taking mass and you're moving it up in the gravitational field, and if you just calculate [laughs] how much that's going to cost—yeah.

ZIERLER: So absent reversing gravity, there's no way out.

WENNBERG: [laughs] There you go. I hadn't thought about that one.

ZIERLER: [laughs]

WENNBERG: "Let's stop gravity, and then we do this." [laughs] "Just for a minute. Get rid of the CO₂ and then—" Yeah.

ZIERLER: Where are you optimistic? What are the things you're hopeful for on a generational level—kids, grandkids, that kind of thing?

WENNBERG: I think the kids are all right. You talk to students these days, even the generally apolitical kids at Caltech, and they all care, and they've come to understand that they want to be part of solutions in this space. So I do see the potential for innovation. If the smartest kids doing physics were in the last generation mostly interested in finance, I think we now have a new generation who's really interested in addressing the climate challenges, whether those be through finance, or device engineering, or through science. I really do see that people are becoming much more engaged. I think that idea that the signal of climate warming is now so far outside the noise, so everyone sees it and knows it's happening, in a way that I think they can understand. The trick is just to not allow pessimism to overwhelm—

ZIERLER: Because that leads to inaction.

WENNBERG: Yes. That's why I'm really excited about where we are right now in terms of the economics of these solutions. Because it leads you to an optimism that I think is going to be helpful.

ZIERLER: Because climate science is so extraordinarily complicated, what do you see as your areas of expertise that lead to clarity? There are so many different kinds of scientists studying so many different kinds of things. What's your lane or lanes that leads to the clarity on the topline issues of what's happening?

WENNBERG: I am an observational chemist and observational scientist. I like to think of myself as being able to visualize data in a way that is a skill that isn't always present in people who are just doing simulation science. Just being curious about data is I think a skill, and a way to have impact. I am an analytical chemist, but one who wants to work on things that are related to improving the environment.

The Gains from Simulation and Machine Learning

ZIERLER: Is simulation useful for you?

WENNBERG: For sure. We have a paper now that we've been trying to get published about not our own, but other people's simulations of the trajectory of these forests that I mentioned. We're trying to compare these simulations with data and try to understand what that tells us about their deficiencies in their models. Because in the end, you do need a model, and that's what we're trying to create. We're trying to create a synthesis of our knowledge that will be embedded in equations in a model. But I think this first-order question of looking at data, trying to understand what it's telling us directly about the way the planet works—how would I put this? One of the things that I've learned throughout my career is that when models are wrong, it tends to be not because a number in the model was wrong, that something was "4" rather than "2," but rather that they didn't know that that number was important. It's the challenge of knowing when you have, at some level, a complete description. In the case of the stratosphere, that was really what was highlighted by all the work we did. The observational science ended up being the key way in which we learned we were missing things. Like, "Whoa. The stratospheric ozone hole. Where did that come from?" It was not in any model. You wouldn't get it through any Monte Carlo error analysis of an existing chemistry climate model for the stratosphere. It was because there were processes that we didn't know existed. That's just one example, but there were many. I think that's true generally—that we have a really complicated system, the natural environment of Earth—

ZIERLER: Which doesn't care about your equations.

WENNBERG: —which doesn't care about your equations. It's by using observations that we can start to really make sure that we're adequately describing not just today, but what happens in the future. One of the examples that I teach in terms of ozone chemistry is that there's this neat figure of what happens to stratospheric ozone when you fly a bunch of Concorde aircraft, the supersonic transports, and what happens to stratospheric ozone when you dump a bunch of CFCs in the atmosphere. The figure relates this to the year in which the calculation was done. What you see is this complete instability, where one year, it's really bad to fly airplanes, but not bad to dump CFCs. Then, we discover something new about the chemistry, and this flips – CFCs bad; airplanes not so much because it turns out these two are generally anti-correlated. Back and forth, and back and forth. It is only stabilized by atmospheric observations that tell us about the underlying chemistry and how it relates to that. You need not just to be able to predict, say, the distribution of ozone today, but you have to be able to predict the tendency of ozone with respect to these perturbations. There are ways to do these kind of things, if you have the right kind of observations and clever people to analyze them.

ZIERLER: Are you dealing with a level of data that computation and machine learning is becoming useful for you?

WENNBERG: Yeah. I'm not good enough, and I'm a little—but my students [laughs]—

ZIERLER: They're teaching you?

WENNBERG: Yes. [laughs] Exactly.

ZIERLER: That's a generational issue.

WENNBERG: Yes, it is.

ZIERLER: What are the kinds of things that machine learning, what are the kinds of signals that are really coming out from the noise, where this is a great tool for the kinds of questions you're posing?

WENNBERG: I'll just give you one example. We've talked about oil and gas. This is not work in my group, but this is the idea. You start to get enough data coming down from this remote sensing of the surface of Earth. You're looking at this column amounts of methane above the Earth. In the old days, what we would do is we would go through these images, and we'd say, "Ah, there's a plume there." We would over the next month analyze all the data taken yesterday [laughs] and find plumes. The rate at which the data is coming in now, these old-school, by-hand look-through-stuff is just not gonna work. This is a perfect example where AI and data science tools where you can set up—you get a nice training data set, you've got your observations, you've got a bunch of experts who go around and say—and now learn how to do this, and now you get real-time, nearly real-time. So, today, there will come a file that will tell us where all the big leaks were yesterday. And that's actionable, because now you actually have the ability not just to say, "Oh, there was a big leak yesterday," but you have the ability to tell that operator, "Hey, you have a leak! [laughs] Do something about it." It just speeds things up in a way that is what we need to do.

ZIERLER: On that point, thinking about your recent postdocs and graduate students, what are the kinds of things that they're doing that might provide a window onto where the field is headed, in atmospheric chemistry and climate science?

WENNBERG: That's really interesting. My students have ended up going in all sorts of directions. Krystal Vasquez is pursuing science journalism; we've had several students decide that that's what they want to do. Some went into policy. You know the Congressional Fellows program? There's also one in California. One of our recent graduates is now a Congressional Fellow in Sacramento, another Ariana Tribby is working for the EPA. I love the idea that these just incredibly technically strong people are able to give technical advice to try to make things better.

ZIERLER: The world needs that, too.

WENNBERG: They really do. Then on the science side, several of my students have stayed in academia, so they're pursuing similar kinds of research to what we do here. Others have gone more into—so like the MethaneSAT project, some students have gone that direction. They want to be part of a company that is making these changes. Again, it's very technical work, but it's the work of very applied science. One of my former postdocs, David McCabe, is at the Clean Air Task Force, which is another one of these big NGOs that has been working on methane for a long time. Alex Teng is a founder of a VC company working on climate mitigation technologies. So, it's diverse. They're all applied scientists, I have to say. They really are.

ZIERLER: What about big industry, working from within the belly of the beast? Do some of your students go that route?

WENNBERG: I have a student I'm just advising now who is finishing up, he works with me and John Seinfeld, who's going to Bain. [laughs] He wants to start an environmental company, eventually, so he's going to go work at Bain. I don't know if that's quite the belly of the beast, but it's belly-adjacent. [laughs]

ZIERLER: It speaks to, at least, an ethos, if not action, of corporate stewardship.

WENNBERG: Yes. That's right.

ZIERLER: Which is generally—absent of when it's greenwashing, this is a welcome development.

WENNBERG: Absolutely. It is where the solutions are going to be implemented, for sure.

The Centrality of Remote Sensing

ZIERLER: Last topic for today, a bit of a thought experiment question. Having spent your research career at Caltech, with all of the ways that it is such a unique institution, what are the areas of research that you think you would have pursued regardless of where you would have been, and what are the things that you've been involved in that you can pretty specifically point to "I got involved in this because I was here, I was at Caltech"?

WENNBERG: I think the whole remote sensing stuff that I've been doing is all Caltech. It's all because I'm here.

ZIERLER: Because it has such a strong history in remote sensing.

WENNBERG: And it's because of who I was around, and the fact that if I look at the rest of the atmospheric chemistry community, who are my age, they didn't do this. Because they are in atmospheric chemistry programs, surrounded by atmospheric chemistry people. I think if I had gone to a more classic institution where you don't get housed next to an atmospheric, ocean, biogeochemist, I don't think that would have happened. Then of course all my friends at the Lab, JPL. Like the planetary stuff. We dropped that; we'll pick it up next time, I guess, the work on Mars that didn't happen.

ZIERLER: What years was that? When was that in the chronology?

WENNBERG: I don't even want to remember. [laughs]

ZIERLER: [laughs]

WENNBERG: But it died during the Obama administration, because of OMB. I almost shut down my whole research program here, because I was the PI of a Mars orbiting—FTS. We were going to do the first complete survey of the trace gases of the Martian atmosphere, which was terribly exciting. And then—yeah.

ZIERLER: On the discovery side.

WENNBERG: That was just total discovery, right.

ZIERLER: Getting involved in remote sensing, what has that meant for the science? What have you discovered? What's known now as a result of this new path that you found yourself on?

WENNBERG: The ability to globally map the sources and sinks of carbon dioxide. It may sound very, very technical, but it's—

ZIERLER: It's a BFD, as some would say.

WENNBERG: It is. It really is. It's the way we start to know if we're making [laughs] progress. Then the second part of being able to literally see the forests for the trees, now. When you stand back and you look at the Earth from space with these new observational tools, and you're able to just pull out of it some very simple solutions to why the forests grow the way they do, that's really amazing. If you read the ecological research around forest ecology, it's fascinating, and it's all this plot-based stuff, where people have gone in, year after year, and measured trees, and try to learn about hydraulics, and lots of other aspects of the ecology of pests, and everything else. But then when you go to space [laughs], and you see something that's just amazing, which is that, if you tell me the growing season temperature, mean temperature, I can tell you, in a pretty statistically strong way, who's growing, and how productive they are. I only need to know one thing, across the whole high latitudes. So, all that other stuff, is interesting, but it's the noise on what is probably the major thermodynamic control of forest productivity. That's pretty amazing, right? It's easy to become a real reductionist, I think. And don't get me wrong, I love molecules and I love to figure out how they all—but the end, when you can stand back and see some of these relationships to energy budget that just show up so simply, it's pretty amazing.

ZIERLER: On that note, we'll pick up next time. We'll go back to Vermont.

WENNBERG: [laughs] Sounds good.

[End of Recording]

ZIERLER: This is David Zierler, Director of the Caltech Heritage Project. It is Friday, February 24th, 2023. It is great to be back with Professor Paul Wennberg. Paul, thank you again for having me. I appreciate it.

WENNBERG: Of course. It's fun.

An International Odyssey Before Vermont

ZIERLER: Today what we're going to do after our initial discussion—it was a great tour of your overall approach to the research, to the science, environmentalism, all of the above. I want to go back today, and let's start at the beginning, with your parents. Tell me a little bit about them, and are they native Vermonters as well.

WENNBERG: No. Well, my father is almost so. But my mom was born in Italy, to a Jewish family, in 1934. In 1939, they got one of the last boats out, and ended up in Ecuador. So, my mom grew up in Ecuador.

ZIERLER: Where in Italy was she from?

WENNBERG: She was born just outside of Turino. Ottolenghi is her last name.

ZIERLER: Did she have any memories of Italy?

WENNBERG: She did, yes, and remembered leaving on the ship, and making their way to Ecuador with her brother. Ecuador was one of the few countries that was allowing immigrants from Europe, refugees, in. You had to say you were doing something agricultural in order to get a visa, and so my grandfather claimed to have some relationship—but he started the first pharmaceutical company in Ecuador, and it's still ongoing.

ZIERLER: Oh, wow. Was that his field in Italy?

WENNBERG: He was a chemist, yeah.

ZIERLER: Runs deep!

WENNBERG: It runs deep, although it's funny, it wasn't part of the conversation, but yes, it does run deep.

ZIERLER: Your mom's family, were they Italian Jews going back a long way, or were they originally from Poland or something like that?

WENNBERG: No, no, they were longtime Italian Jews. Somehow related to Segre, as well—Emilio Segre, the nuclear physicist, who was related to my mom's mom. So, my mom ends up in Ecuador. She grows up there. Her mom dies in childbirth with her sister a few years after they get there, so she and her sister and brother grew up together in a really tight-knit family.

ZIERLER: Did your grandfather remarry?

WENNBERG: He did, but only after his kids had left. He was a strong proponent of education, so he set his kids up in an apartment with some cousins, in Montreal, to go to high school. So, my mom was the sort of elder, at 16, was the oldest person in this household, of kids, living in Montreal and going to school there. So, she did that. She did a bachelor's at McGill, eventually, and then became—in biochemistry—and then she went to medical school at McGill.

ZIERLER: Growing up, did she speak Spanish and Italian?

WENNBERG: Yes. And English. And then French.

ZIERLER: Did they retain any Jewish identity in Ecuador? Do you know if they were affiliated in—?

WENNBERG: They were affiliated with the synagogue. There was a small Jewish community there. But my mom was never a practicing Jew, really. That wasn't really part of my cultural heritage, other than—we did have Passover, but it was liberation theology Passover.

ZIERLER: Very good! [laughs]

WENNBERG: She goes to medical school.

ZIERLER: In Montreal.

WENNBERG: One of the few women of course, in the time. I think she said there were two women in her class. But she meets my dad there. I still don't quite understand why my dad went to McGill. He grew up all over the place. His father was a paper chemist, so another chemist, but very much applied, in the paper industry. He worked for Weyerhaeuser, so they moved around a lot. He did live in Vermont as a kid for a little while, but he also lived everywhere else.

ZIERLER: Where there's lots of trees.

WENNBERG: Lots of trees. And I think if you ask him, he would say he grew up mostly in the Pacific Northwest.

ZIERLER: So, they met at McGill.

WENNBERG: Yes. He went to Stanford, undergrad.

ZIERLER: Was he getting an education at McGill, or he was just hanging out there?

WENNBERG: No, he went to medical school. They both—that's how they met. He went to medical school at McGill. I don't know why, what led him to go to Canada for—

ZIERLER: Generationally, it's not the draft. That's too early.

WENNBERG: That's too early, yeah. Although we get to that, later. They met, got married in Montreal while in medical school, had my older brother. So, my mom is going to medical school, also having my brother.

ZIERLER: Is your dad from a Jewish background also?

WENNBERG: No, Episcopalian or something.

ZIERLER: Okay, Wennberg is German?

WENNBERG: No, it's Norwegian.

ZIERLER: Norwegian.

WENNBERG: Yeah. My grandfather, my dad's father, came to the United States in the twenties from Norway to study paper chemistry. He was a bit older, as an immigrant, certainly than my mom had been.

ZIERLER: Did your parents have a two-body problem when they finished school?

WENNBERG: Medical school is one of those things where—

ZIERLER: You can just kind of pick up a job?

WENNBERG: Yeah. So they went to Washington D.C., first, for their—whatever it's called.

ZIERLER: Residency?

WENNBERG: Yeah. And I was born there. Spent my formative—like six months there. Then we moved to Baltimore. My dad went to Johns Hopkins for a fellowship. My mom worked. My dad studied public health there. But he never—he ends up never practicing. So, he goes through this whole medical thing, and never, ever actually deals with patients after—

ZIERLER: Because he was into more public policy stuff?

WENNBERG: Yeah, public policy. He escapes the draft by getting a job in Vermont. We moved there in 1966, so I was four years old.

ZIERLER: What was the job?

WENNBERG: He worked for the state of Vermont, to do healthcare planning for the state. The state was trying to figure out how to—how many imagers do they buy? Where do they put them? This is classic sort of rural allocation of resources. So my dad starts this job, and then he realizes that medicine is completely irrational, how resources are allocated, and then you can use this as a natural experiment to learn about what happens if you go to a place that has, say, a physician who likes to remove tonsils, and one that doesn't, in an area nearby that looked completely the same except that one has this doctor who takes out everyone's tonsils. So, he makes a career of this, looking at the way American medicine is practiced in the field, and what it tells us about whether it's useful.

ZIERLER: And the kids?

WENNBERG: Then there's four of us kids.

ZIERLER: Where are you in the order?

WENNBERG: I'm second. My younger two siblings were born in Baltimore, before we moved. When we moved to Vermont, to this farmhouse, cold farmhouse in Waterbury, there were four of us, all very, very young. My mom [laughs], she was working full-time, all the time, and she starts out working at the University of Vermont, in pathology, so she's more on the—you were trying to find a job that actually had a schedule, right? But then she helps found the Vermont Women's Health Center, which is the first abortion clinic in Vermont. This was before Roe v. Wade. She really moves full-on into women's health and then opens her own practice eventually in Waterbury. Then she had a second career. Right after my youngest brother went to college, she moved to Bolivia to start a rural clinic in a place that was without anything. So, she sells her practice to Planned Parenthood and moves to South America.

ZIERLER: Solo?

WENNBERG: She moved with a friend, a couple actually, so they had—a midwife friend of hers, they decided to go together. But my mom was the Spanish speaker. So, they go to this middle of nowhere place—it's just unbelievable—and she works there for three or four years, and then she ends up becoming involved in Third World healthcare, from planning and practice and everything else. So, she works for the WHO. She spends a couple years here, a couple of years there. Guatemala, Nicaragua, Dominican Republic, South Africa. She travels all over and lives in these places. She worked in India for a while. Yeah, so she has this nomadic second career, before moving back to Vermont.

ZIERLER: For you, your upbringing was mostly in one spot in Vermont, or your family moved around?

WENNBERG: We moved to this house, and we stayed there.

ZIERLER: Where was it?

WENNBERG: In Waterbury Center.

ZIERLER: This is a rural kind of area?

WENNBERG: Very rural. So, my bus ride to high school was almost an hour.

ZIERLER: Oh, wow.

WENNBERG: Yeah, it was really out there.

ZIERLER: This is like a cabin in the woods kind of situation?

WENNBERG: Not quite, but there were—when you went—as a kid, if you'd go trick or treating, there were five houses to go to, they all knew you were coming, and they had made exactly the right number of pieces.

ZIERLER: No surprises. [laughs]

WENNBERG: [laughs] It was a pretty rural experience, and I grew up on what used to have been an upland dairy farm. My parents were sort of hippies. My dad was a good—he was sort of a—what would I say?—he had lots of ideas, and was not into execution.

ZIERLER: Like a dreamer?

WENNBERG: Yeah. And my mom could execute, but also it landed on us kids to do most of the execution of my father's vision for how we should organize our lives. Like the acre of gardens, and the whole—so, the kids [laughs], we always joke that it is a bit surprising that we made it to adulthood and survived.

ZIERLER: [laughs]

WENNBERG: We were doing some pretty crazy stuff, as kids.

ZIERLER: Did your family keep animals?

WENNBERG: Oh, yeah. We used to board horses in the winter, for people. We had chickens, sheep.

ZIERLER: How would you describe your parents' politics? Like, way left of center? Did they talk about politics?

WENNBERG: We did talk politics. But I wouldn't say—well, my mom was—I would say she was a very strong—she was a really strong early feminist. A lot of left-wing politics back then, of course, was not particularly feminist, if you go back and you read this history. So, she didn't—we didn't really—and you're living in the middle of fricking nowhere, so it's not like there was a—

ZIERLER: There's no internet.

WENNBERG: Yeah. So I wouldn't say politics was—we did talk about politics, but it wasn't really the core.

ZIERLER: And Jewish practices, nothing?

WENNBERG: Nothing.

ZIERLER: Like zero.

WENNBERG: No religious—

ZIERLER: You knew your mom was Jewish.

WENNBERG: Yes. No religious practice.

ZIERLER: Nothing.

WENNBERG: Nothing.

ZIERLER: No Christmas.

WENNBERG: Nothing. Well, we had Christmas, but it was trees, and decorations. But there was really no religion. I didn't grow up religious at all. My parents were atheists, and yeah, I don't think—it didn't have anything to do with how we grew up.

ZIERLER: Were you aware that you were Jewish, like in the sense that your mom is Jewish?

WENNBERG: I knew that my mom had Jewish heritage. We had lots of visitors of her family from all over the world, so you understood from them—my grandfathers would come from Ecuador, or cousins coming from Italy, from Montreal—that being Jewish was important to them. Her sister is very Jewish, her younger sister.

ZIERLER: What was your school like? Tiny, I assume.

WENNBERG: My school?

ZIERLER: Yeah.

WENNBERG: The elementary school was tiny, but the middle school and high school—that's why the bus ride was so long—because they built these—

ZIERLER: They pulled kids from all over.

WENNBERG: Exactly, yeah. So I think there were probably 70 in my graduating high school class. That's still small, but—

ZIERLER: Sure. It's not a one-room schoolhouse.

WENNBERG: It's not a one-room schoolhouse, yeah. And it wasn't a great high school. It was fine. [laughs]

ZIERLER: Did the environmental movement of the 1960s and 1970s register with you?

WENNBERG: Very much, and with my parents, too, a lot.

ZIERLER: Earth Day, the United Nations Environment Programme, that kind of thing?

WENNBERG: Yeah. Well, before Earth Day, there was a celebration in Vermont that was called Green Up Day, which was more of a like collect-the-garbage kind of movement. We were big involved. Vermont was a hotbed of environmentalism. They passed a bunch of laws when we were kids to—there's no billboards in Vermont, for example. They passed one of the first container recycling bills, so if you buy a beer, it had—you'd get five cents back when you took the bottle back. That was very much the kind of environmentalism that we were exposed to as kids.

Tinkering and Early Ecological Interests

ZIERLER: Would you say you were oriented towards science growing up?

WENNBERG: Yeah, I would. And technology. Growing up on the farm—

ZIERLER: We talked about tinkering last time.

WENNBERG: This is what I did. I was a tinkerer, and fixed cars, fixed stuff. I was the one who knew how to fix the hay baler and the tractor and stuff. And my uncle was a biochemist, and he was actually a practicing scientist, pretty much the only one I knew.

ZIERLER: Where was he?

WENNBERG: He was in Denmark. He would come to visit fairly frequently, every other year, and we had lots of fun talks. He was researching the sodium pump.

ZIERLER: Were you aware of things—certainly wouldn't have experienced it in Vermont—but like smog in L.A. and industrial pollution? Did you think about that stuff growing up?

WENNBERG: Not so much air chemistry. It really wasn't a thing. But water pollution, because water pollution is everywhere, including in the rural areas. And I remember like when the Cuyahoga River caught fire, that kind of reporting. Pretty dramatic.

ZIERLER: "This is not right."

WENNBERG: Yeah. But we did—my dad's dad lived here in Southern California, and so I did experience the pall of air pollution when we came to visit. It was really stunning. Because you come here in the 1970s, and first of all, the freeways, right? You've got to remember, you're coming from Vermont, [laughs] and you arrive here, and there's these ten lanes in each direction.

ZIERLER: Did that plant a seed, do you think, when you experienced the smog?

WENNBERG: Maybe. I was interested in environmental chemistry. I definitely was, as a kid. But I don't think atmospheric chemistry—chemistry of the air was not really in my focus at the time, at all.

ZIERLER: Was your high school big enough to offer AP classes?

WENNBERG: [laughs] No.

ZIERLER: Nothing like that?

WENNBERG: No. We didn't have calculus, either.

ZIERLER: Didn't have calculus.

WENNBERG: No. It's funny; [laughs] the reason I go to Oberlin is because I had this math teacher, Mr. Kaufmann, Donald Kaufmann. They had what was called precalc, which is basically algebra. I was really annoying. He really—

ZIERLER: Too many questions.

WENNBERG: Yeah, or just being a smart aleck or whatever. So he kicked me out of class, and so I had to go to his cubby in the teacher area and he gave me a calculus book. So, I studied calculus at his desk, while he taught. So. But yeah, there were no AP classes, at all.

ZIERLER: Going to UVM, would that have been in the cards?

WENNBERG: [laughs] The guidance counselor at school was the driver's ed teacher. So, you met with him and he said, "Okay, there's Saint Michael's and there's UVM. Those are your options." [laughs] "That's what you should do. Go to one of those two places."

ZIERLER: Oh, I meant go to UVM, like take classes there, as a high school student.

WENNBERG: Okay, I see what you're saying. I had no idea if that was even possible. It was never a part of the conversation. I did play in the UVM Orchestra as a kid.

ZIERLER: What's your instrument?

WENNBERG: Bassoon. I was really one of the best bassoonists in Vermont.

ZIERLER: Really!

WENNBERG: [laughs] I was the only bassoonist in Vermont!

ZIERLER: There you go! [laughs] Did you keep it up?

WENNBERG: All the way through until my older daughter was born, and that just—yeah, that was the end. But I played in the Harvard Radcliffe Orchestra. That was the other reason I go to Oberlin, right? My math teacher—

ZIERLER: Strong conservatory there.

WENNBERG: Yeah. My math teacher says, "Paul, you should go to Oberlin, because you like science, and they have really good science, and you like music, and they have really good music." So that's what I did. I went to Oberlin. Because he told me about it.

ZIERLER: I could see the Vermont to Oberlin transition a pretty easy one.

WENNBERG: But I never had heard of it. And I believe I was the first person ever, from my high school, to go to Oberlin.

ZIERLER: What year did you get to Oberlin?

WENNBERG: 1980. In the Fall of 1980. I finished high school early, and then I went to Europe. I left high school in December and went to Europe and traveled around for six, seven months, eight months, then came back and went to Oberlin.

ZIERLER: On your own?

WENNBERG: Yeah.

ZIERLER: Fun time?

WENNBERG: Oh, it was great. You learn a lot traveling by yourself. Not always fun. But it was mostly fun. And you know, you run out of money, go pick oranges, kind of—that was the time where you could pretty much get a job anywhere in Europe, doing something menial, to get enough resources to go to the next place. I did have a plane ticket home, so [laughs]—

Chemistry at Oberlin

ZIERLER: Were you purely musically oriented at Oberlin? Was that the main game plan?

WENNBERG: No, no. I was nowhere near good enough. I would never have gotten into the Conservatory there. But I did play. I played in the like second orchestra that they had, also in chamber groups and so forth. Took lessons. But I went there to do science. I did chemistry at Oberlin.

ZIERLER: How did you come to chemistry?

WENNBERG: I don't know. I think it was because my uncle was a chemist. It never—I didn't think of anything else, I'll say.

ZIERLER: Was it a strong program in chemistry, looking back?

WENNBERG: Very strong, yeah. They had some just really good people there. And it was known at the time as being one of the best predominantly undergraduate institutions doing research.

ZIERLER: So the professors there were engaged in research.

WENNBERG: Very much so. And they taught the undergrads how to work doing research.

ZIERLER: And you're not competing with graduate students for their attention.

WENNBERG: Exactly. So it was very much—and I did some analytical chemistry research there. One of the key people, though—I didn't work with—was Norman Craig, but he was my freshman chemistry teacher, and we stayed friends throughout his life. He was an amazing guy. He just taught a whole host of people who have gone on to academic careers. He taught them how to do chemistry research. There was a time when graduates of the chemistry program at Oberlin had an enormous number of academic positions. It was like way, way, way outside of what you could ever have imagined.

ZIERLER: This is after, of course, the Vietnam era, but was campus political? Did it feel political?

WENNBERG: It was. Because Oberlin is always political. It's funny; Oberlin is in Oberlin, Ohio, and it's a classic liberal arts college that owns the town, sort of. But there was also a strong—

ZIERLER: Town-gown?

WENNBERG: Yeah, because—so, just north of Oberlin is Elyria, so there's a large Ford factory. So, there were a lot of people who worked in the automobile industry assembly. So, it was an interesting group. You have the college, but then you also had lots of folks who worked in the automobile industry that were living in Oberlin. But it's a teeny town. So, I go from Vermont to Oberlin, and I'm going like, "Oh my god, there's so much to do here." It was like you go from this little teeny neighborhood with like five houses, and you go to this place, and there's movies every night, and there's concerts, and there's—it was just incredible. Then all the kids who come from New York City or from California, they're going, "Oh my god, there's nothing to do here!" [laughs]

ZIERLER: It's all perspective.

WENNBERG: It was, yeah. So, I go, and I studied for two years, and then I started to think I wanted to maybe do something—this is where I got interested in air chemistry—in a field called industrial hygiene, which is essentially public health of the workplace, associated with air in this case. You can think of it as like exposure chemistry. What are people being exposed to, and how does it influence their health and so on? I decided I should probably try this out, so I went—took a year off, and I worked—

ZIERLER: This would have been your third year?

WENNBERG: Yeah, and I worked at the Harvard School of Public Health, in the Analytical Chemistry Labs there. It was a fun experience, sort of.

ZIERLER: So you dropped out of school or you took a leave?

WENNBERG: Just took a leave. And I went to work, and I was doing research there. This was very much a research lab. Jack Spengler and Tom Smith were doing exposure assessment —so I got to ride on like railroads, go and ride in the cabins of locomotive trains and measure the amount of diesel exhaust that people were being exposed to. We had this project in Canada, which was a silicon carbide factory, so basically coking, where they would do electrolysis of coal, and just a horrific place, measuring both particulate and sulfur exposure. So I did that for a year. I liked the research side, but I really didn't like much the talking to people about health. But I sort of put that aside, and went back to Oberlin, finished up.

ZIERLER: This helped you realize you were more on the science side?

WENNBERG: Science-y side, yeah. After I go to finish my degree at Oberlin, I go to work for a year at the, again, back in Cambridge, at Harvard, in the Safety Office—if you think of these guys over across the street here who come and check out that your fume hoods are working and that kind of thing. Very boring work. Also, I realized directly how irrational the conversation is around people when they're talking about their own health. We just get afraid. We stop being rational people that we might think we are. And I really didn't enjoy it at all. So, I decided, "I think I should do something very much in academia, where you don't have to talk to people." [laughs] So, I decided I'm going to go and—at the time, what was really interesting and important at the time was the question of stratospheric ozone. So, I thought, "I would really like to contribute to that science."

Early Concerns Over the Ozone Hole

ZIERLER: So the alarm bells on the ozone hole were already ringing at that point.

WENNBERG: Oh, yes, yes. This was actually when it was really happening. 1985, Joe Farman publishes this paper. I'm at Harvard in the Safety Office. People had started worrying about chlorine chemistry in the 1970s. So, Rowland and Molina publish their paper in the 1970s about CFCs. There's a bunch of work going on about how the halogens were changing the ozone. But then the ozone hole appears in 1985 when I'm looking at graduate school.

ZIERLER: It's not that it appears; it's that it is discovered.

WENNBERG: It is discovered. Although, yeah, it appears—it actually starts showing up in like 1979. So it really goes—it's a precipitous—

ZIERLER: Oh, wow.

WENNBERG: —onset of halogen-driven chemistry.

ZIERLER: Is that because these chemicals are only being pumped into the atmosphere relatively recently?

WENNBERG: Yes. And also it's incredibly non-linear. Because it goes roughly as the square of the amount of these chemicals. So, it really was like a classic canary thing, where all of a sudden, big ozone loss over Antarctica. We didn't even know it was halogen-based, but there was a strong suspicion. In fact the chemistry didn't work out at all. You would never have anticipated the ozone hole. So it was very much an observational—

ZIERLER: What were some of the early concerns? Obviously there's a hole; that's not good. But not good how?

WENNBERG: The ozone layer—the stratosphere of ozone removes and screens the UV from the surface of Earth.

ZIERLER: So this is like a skin cancer epidemic?

WENNBERG: It's not just skin cancer. That's certainly for people. But it's the entire UV dose. This is the dose that causes mutations of all biological materials. So, not good for plants, not good for animals, not good for humans.

ZIERLER: Dystopian hellscape kind of thing.

WENNBERG: Yeah, it really seemed like, wow, if this thing spreads out from Antarctica, where it's dramatic but the impacts on the surface are fairly small—because there's just not much life there—but if this chemistry had proceeded the way it was going, you could really have huge damage to the planet.

ZIERLER: What were some of the early theories as to why Antarctica? Why would it concentrate there? Is that an air pressure thing? Is that a temperature thing?

WENNBERG: The dynamics are unique there. You do end up with a much colder stratosphere there than you do elsewhere.

ZIERLER: Even the North Pole?

WENNBERG: Yeah. We don't have time to really go into this, but the wave dynamics that stir the stratosphere are really different in the north and the south. In the south, you have the Southern Ocean, so you don't have this distribution of oceans and land that you have in the Northern Hemisphere. That produces, in the Northern Hemisphere, a much more—what's called Rossby waves—but basically the large-scale turbulence of the Northern Hemisphere is quite distinct because of this swirliness that you get driven by the thermal contrast between the oceans and the land. And that really changes the dynamics of the stratosphere. It's a lot warmer in the North.

ZIERLER: The chlorine, the CFCs, is it like a magnet that is attracted to Antarctica?

WENNBERG: No. There's the same amount there as there is in the Northern Hemisphere. It's that the temperatures get cold enough that you form ices, and on the ice, you get heterogeneous chemistry that liberates the free radicals that then essentially chew through the ozone in about a month.

ZIERLER: So if it does have this unique climate, why would then there be a concern that this could happen elsewhere?

WENNBERG: We now know that the halogens were chewing away at ozone everywhere. They weren't doing it quite as dramatically.

ZIERLER: So this is the canary effect, where we're seeing it here, it's just happening faster here, but ultimately if we keep doing this—

WENNBERG: Yeah. Just to realize how bad these CFCs were, if we continued on the production increases that were going on at this time—CFCs are wonderful chemicals; they're used for refrigerants—but if we had kept on the trajectory we were going on, they would have been a larger component of global warming today than CO₂ would have been. So, these CFCs are remarkably strong greenhouses gases, in addition to their chemical influence on ozone. There have been these suggestions of terraforming Mars, and they all involve figuring out how to set up a factory to make CFCs on Mars and release them to the air.

ZIERLER: The Martian hairspray factory.

WENNBERG: Yeah. They're really, really good—they're really good greenhouse gases, yeah.

ZIERLER: Was anybody at this time, in the early to mid-1980s, were they also thinking about global warming? This is before James Hansen, 1988, but people knew, people were making these connections.

A Circuitous Path to Graduate School

WENNBERG: Okay, so, let me tell you how I get to Harvard graduate school, because that's not obvious.

ZIERLER: You're already at Harvard, but this is a staff position.

WENNBERG: It's a staff position, and I hate Cambridge. I really don't like Cambridge, so I'm not staying there. I apply to study at University of Colorado, Boulder, which had an association with NOAA Laboratories, there, where a lot of this research was going on. I apply, and I get in. And then I get—must have been a letter or a phone call—email, maybe?—I don't know, that seems a little early—so the scientist I was going to work with decided he was going to retire, and he wasn't going to take any more students.

ZIERLER: He was a staff scientist at NOAA?

WENNBERG: At NOAA, yeah. And so, I'm like, "Fuck."; "Okay, I'll just wait a year and I'll apply again somewhere else." I had a friend of mine from Oberlin, Phil Stevens, who worked in Jim Anderson's group at Harvard which also did stratospheric research. Phil says, "Paul! The yield this year in Chemistry graduate program was really low, and they don't have enough teaching fellows. Go talk to the head of the admissions committee. Maybe they'll let you in." So I go talk to Dave Evans, who used to be on the faculty at Caltech—and he happened to be one of Norman Craig's students from Oberlin. So, I go up to him and I tell him my sad little story, and he says, "Let me call Norm and I'll get back to you this afternoon." That's how I went to Harvard.

ZIERLER: [laughs]

WENNBERG: I never applied.

ZIERLER: No application? They just—?

WENNBERG: No application. Just let me in. But it was because they needed teaching assistants. My life has been very much of a stumbling through lucky.

ZIERLER: Just as a fun thought experiment—had you ended up at Boulder, would you have pursued more or less the same kinds of topics?

WENNBERG: Who knows, right? Who knows. But I was geared towards atmospheric chemistry.

ZIERLER: That would have still happened.

WENNBERG: Yeah. Anyway, that's how I end up at Harvard to do my PhD work.

ZIERLER: The department is Chemistry?

WENNBERG: Chemistry. Jim Anderson actually was trained as a physicist, but he was hired into the Chemistry program there.

ZIERLER: How far back does atmospheric chemistry go at Harvard?

WENNBERG: Okay, so this is interesting. Because you asked about global warming, which this brings me up to. So, Michael McElroy—I think we would call him an atmospheric chemist, but he was trained in the geosciences, so he was not a chemist-chemist, at least I don't think he trained in chemistry. He's still alive, still working. He was the person who introduced me to global warming. He had a class for undergrads interested in the atmosphere who were not going to be atmospheric scientists. So it was like the classic—

ZIERLER: "Physics for Poets."

WENNBERG: "Physics for Poets." This was "Atmospheric Science for"—

ZIERLER: —Poets.

WENNBERG: Whatever it would be. Poets. Yeah. And so, I was a teaching assistant for this class, and it was a great class, and he taught about climate. Then Al Gore came. He was a friend of Al Gore's. So, Al Gore came. This was when he was in the Senate. And really from very, very early on, Al Gore was very interested in global warming.

ZIERLER: I was going to ask, where is Roger Revelle in all of this?

WENNBERG: I don't know.

ZIERLER: This is before your time?

WENNBERG: Yeah. I remember distinctly going to hear Al Gore and Mike McElroy at this symposium that they had at Harvard, in I think it was 1985 or 1986. I taught with Mike, and we taught about climate, and we taught just basic atmospheric science. That's how I ended up—that was my introduction to climate as a topic of great interest. But my work in the Chemistry labs was almost completely distinct from climate.

ZIERLER: This is the hydroxyl work.

WENNBERG: Yes, I was doing measurements of free radicals in the stratosphere to try to understand ozone chemistry.

ZIERLER: Specifically because you wanted to be part of the solution to the ozone hole?

WENNBERG: Yeah. I was interested in—solutions; I don't know. You gravitate—

ZIERLER: It's basic science, but—

WENNBERG: It's basic science, and you're going like, "Oh my god, this is really, really important, to figure this out." And it's very, very exciting to figure this out, because you're part of something that's immediate.

Jumping Into CFC Research

ZIERLER: Where in terms of the knowledge base—there was already the obvious causation understanding between CFCs and the ozone hole, but what were some of the open scientific questions at that point?

WENNBERG: The part that I worked on was much more about the halogen chemistry or the photochemistry of the mid-latitude stratosphere, so this is where people live, and how halogens and other things are organized. There had been these longtime numbers of calculations that people had done. For example, what happens if you emit CFCs to the atmosphere? And, what happens if you fly supersonic transport planes? Because they dump a lot of NOx in the stratosphere. Famously, Hal Johnson, who was a physical chemist at Berkeley, had testified to Congress that they should not be funding supersonic transports because they dump a lot of NOx in the stratosphere. At the time, NOx was sort of thought to be sort of the most important catalyst for ozone loss. NOx in the stratosphere is prominent, and it comes not from combustion like it does here at the surface, but actually from nitrous oxide. So, nitrous oxide, N2O, is produced biologically, and it gets up into the stratosphere. It's a long-lived gas, and a small fraction of it gets converted to NOx up there. So, the early calculations said NOx was by far the most important free radical, and if you fly planes, you're going to now have this additional source. So it made a lot of sense, if you were to fly planes, put more of the thing that was controlling the loss rate of ozone, you would end up with less ozone. The early calculations suggested this was the case. Part of the stuff that I worked on was basically we now know that the chemistry is much, much more complicated, and that NOx is very important, but in the lower stratosphere it plays a completely different role in controlling how much chlorine is there, and the abundance of hydrogen radicals. So I helped to build a machine to measure the OH and hydroperoxyl radical in the stratosphere. Really it was the first time it had been measured well, from an airplane.

ZIERLER: Tell me about building the machine. How do you start? What is your starting point?

WENNBERG: I joke it's my second PhD, because I was there for a long time. I had done some work, working with others, and we had this opportunity, then, to build a laser induced fluorescence measurement to measure OH and HO₂ from NASA's U-2 airplane. Crazily—

ZIERLER: These are the same U-2s like from the spy issues in the 1960s?

WENNBERG: Yes. Crazily, the PI of the lab, Jim Anderson, decided that I should lead this effort, as a graduate student, to spend what must have been more than a million dollars on this—I don't know how much it cost, but it cost a lot.

ZIERLER: This was NASA- funded?

WENNBERG: Yes, NASA- funded. I think he was either foolish or insightful, but he did end up tasking me to help to lead this project. I worked with some really talented engineers – Nate Hazen, Jim Oliver, Norton Allen, and Larry Lapson - and other scientists including Rick Stimpfle and Ronald Cohen to build this machine. One of the key [laughs]—the lab had built a previous balloon-based measurement of OH radicals, so it was based on some of that work. But we used what's really old—well, now it is considered very old, but at the time it was remarkable—laser technology which is called copper vapor lasers. You actually use copper atoms in a plasma, pulsed with a very high voltage, to produce green light, and then you can pump a dye laser with that, and double that to make UV that you can excite hydroxyl radicals. We had actually—not me, but the lab—had built one of these crazy systems for a balloon-based package. Jim decided he was going to build one for the airplane. We had to build the whole laser system. I don't think it ever would have worked. What happened, though, was as we were starting this project, we realized, or I realized, that diode-pumped YAG lasers were just coming to the—they were sort of more research-y at the time, but there were a few hints that this might be the way out of our technical problems. So, we worked with Spectra-Physics who had developed something they weren't selling at the time, but we managed to get two of their early prototypes, and used them on this airplane system to build. So, we built them into the U-2 to be able to make the light we needed. It was really fun. It was really fun to build stuff.

ZIERLER: Did you ever get to go up in the U-2?

WENNBERG: No, no, no. There's a single pilot. I would have loved to. There is a trainer [laughs]—

ZIERLER: Air Force gets involved, I assume?

WENNBERG: Yes, but in a weird way. NASA has two of these planes, or three now, and they're called ER-2s. They're basically the U-2. But all of the pilots are Air Force alums. Often enough, they retire from the Air Force, and they end up working for NASA flying these airplanes.

ZIERLER: Where is the machine mounted? What does it look like?

WENNBERG: It is mounted in the nose of the airplane, and—what does it look like? The main optical bench was a monolithic system where we put these two—we needed to use two of these diode-pumped YAGs, because individually they were not powerful enough to drive the dye laser. What we did was we—these are pulse lasers, and we realized that you could do a pump amplifier approach, but in the same cavity. So, we built the dye laser as a small thing about 50 by 50 centimeters. Because you're only tuning over a teeny little frequency range, so everything is locked down, and we used just an etalon to tune the frequency on and off resonance with the hydroxyl radical. We pulsed the two lasers. So you'd send in the first pulse, and then a few nanoseconds later, you'd send in the second pulse. So, after you get the cavity above threshold, narrow enough, and then you send in the second pulse, into the same cavity, and you get a pretty good response that you could then frequency-double in one of these magic crystals.

ZIERLER: How was the machine capturing data?

WENNBERG: So then the light goes into the air—so we have air flowing right through the nose—the big hole in the front of this airplane to bring the air into our fluorescence chamber—and then we hit it with this light, in what's called a White cell, after Mr. White, so it's a multi-pass cell. Then 90 degrees from that, we image the fluorescence from hydroxyl radicals. So, you have a PMT—photomultiplier tube—collect the numbers of counts. It goes to a counter. And it all gets recorded onto a hard disk. But like all the things you don't think about—these are mechanical hard disks, and the reader is sitting on a pocket of air as the thing spins. So it does not work at the pressures of the ER-2 flying at 100 millibars. So, everything you think of, like, oh, okay, we have a hard disk, but we're going to have to build it inside of a pressure chamber, because otherwise the disk will crash. So, it was quite a fun engineering problem.

ZIERLER: Where do you tell the plane to go? Everywhere, or are there targeted areas?

WENNBERG: I was involved at some level in the mission planning work as a graduate student, but I became much more involved later. I was there to mostly work on this machine and make it work and record the data and then do analysis. One of the things about atmospheric chemistry research is that it's incredibly collaborative. No one or no one group makes all the measurements you need to understand what is going on. So there were groups from NOAA, there were groups from NASA, there were groups from other universities. There was the Harvard group. We all worked together on this platform. Everyone in the morning comes early, gets everything working, gets it all ready. The pilot comes, and the pilot has been told by the science team of whoever is organizing the mission—in this case, it was Steve Wofsy and Paul Newman —what they want them to do. "We want you to go up to this altitude, we want you to fly over there," and so forth.

ZIERLER: I wonder, is there a military dimension? Do we need to tell the Soviets not to be nervous about this?

WENNBERG: [laughs] We did have a—we'll get, I guess, eventually, to Caltech, but I come here in 1998, and the first thing I do—because sometimes it's like, you want to do new stuff, but you also need to have some cred—and so, I decide I'm going to build a new instrument, but one based on mass spectroscopy. Because, well, mass spec offers a lot that optical instruments don't, namely you can measure lots of things with a single machine, which is really great. Anyway, I come here, and we do this experiment, the first experiment. We build this instrument here, a mass spec to fly on the ER-2, and we go to Sweden, to study ozone chemistry over the Arctic. Again, with the U-2. And the Russians were involved, in the sense that we had a number of Russians who came, and they would—because we were allowed to fly over Russia, with the U-2. Which is pretty amazing, right?

ZIERLER: Yeah!

WENNBERG: But they had to be on the ground and inspect. The NASA guy had it seemed like pockets of cash in his pocket that he doled out to the Russians for their per diem. So, yes, we did have to deal with the Russians. [laughs] But in that particular case it was to—

ZIERLER: You joked that building the machine was the second PhD, meaning that you were really there to do the data analysis as your real PhD, and this was something you took on?

WENNBERG: I worked on the balloon-based instrument, and also did atmospherically relevant kinetics. So I did sort of classic—one of the classic sort of P-Chem things you did as a graduate student when you learned stuff was to do kinetics, where you measure how fast things react.

ZIERLER: By the way, you said you didn't like Cambridge. Were you so fully ensconced in the research that it didn't end up being a problem?

WENNBERG: I guess that's true. Yeah, I still don't like Cambridge. [laughs]

ZIERLER: [laughs] And you weren't in a hurry to leave, either. This is a pretty drawn-out graduate process.

WENNBERG: That's true. Yeah, I was there a long time. I met my wife in Cambridge, and she was—she's definitely an extrovert.

ZIERLER: She was in school at the time, also?

WENNBERG: She had just finished her masters in public health at BU. She had a number of jobs, subsequent jobs, but she had a huge group of friends there, and was not interested in leaving Cambridge, at all. And I was enjoying my science, and as you say, that gets you around a lot of the things you don't like, just that you love to go to work and love doing the exciting stuff we were doing. So, I was just happy to hang out.

ZIERLER: Are you keeping tabs on the Montreal Protocol? Is this—?

WENNBERG: Oh, yeah.

ZIERLER: Is it more than interesting? Are you involved at all?

WENNBERG: The Montreal Protocol exists in this ecosystem that involves assessment, and the assessment, which is echoed by the IPCC today—and in fact, the IPCC assessments had many of the same people involved. So, I was involved in those assessments, helping to write them and review them. But they were very much not on the policy side. This was like these books of, what do we understand about the chemistry of ozone. I was assisting with that. Yeah, it was definitely part of—but the politics side, I was not involved.

ZIERLER: How long was your graduate experience for, in the end?

WENNBERG: [laughs]

ZIERLER: You started in 1986?

WENNBERG: I started in 1986; that's right. I finished in 1994. But I stayed on afterwards. I did kinetics, and then I did balloon-based observations, and then normally, when you would graduate, so after five years, I'd start this whole new project on the ER-2, which takes me up into 1997 and 1998 field campaigns.

ZIERLER: This is a long time to be in Cambridge. [laughs]

WENNBERG: It was a long time, yeah. But then I stayed on as a staff scientist.

ZIERLER: By 1993, 1994, such a long experiment, how did you know when you had enough to put a dissertation together?

WENNBERG: I was not [laughs]—

ZIERLER: Did your advisor tell you to like wrap this up?

WENNBERG: No, not at all. Jim was a great advisor in the sense that he provided a wonderful place to work, really talented engineering staff, an amazing group of other graduate students who were there when I was there, who have all gone on to do really amazing things. My contemporaries in Jim's group make up a ‘who's who' of atmospheric chemistry. Jon Abbatt, Darin Toohey, Linnea Avallone, Ronald Cohen, Neil Donahue, Hope Michelsen, Phil Stevens, Andy Dessler – all working together in one place. It was one of those magic times where you're in a group—

ZIERLER: And the funding was good?

WENNBERG: The funding was good. But I met individually with Jim about once a year.

ZIERLER: Oh, wow. That's really hands-off.

WENNBERG: Yeah. Once a year, you gave a group meeting and then you had a little chat with him. That was sort of the level of direct mentoring.

ZIERLER: Is this to say that his own research was not so intertwined with what you were doing?

WENNBERG: Oh, no, no, no. [laughs] It was so—

ZIERLER: It was.

WENNBERG: Yeah. It was just his style. He was very much of the—"You're here to develop independence, and the way you're going to do that is by being independent." I may be exaggerating a little bit, but we did not meet with him very often. It's quite different. It's not the way I treat my students. But it was an interesting model. Then of course because Harvard being Harvard, where groups don't collaborate very much, it felt to me like that was the only model I really knew. I didn't have a strong social network of people in academia, so it just felt like that's the way it was. It was interesting when I came here, and this is a much more collaborative environment, to see, "Oh! You don't have to have it that way. This can be a much more—" Harvard just had a set of—the way the institution is organized is much more, each boat is a separate boat. And if you collaborate, that's almost a sense of weakness, you felt, at the time.

ZIERLER: You could have defended earlier just in terms of how much data you had?

WENNBERG: Oh, yeah. I was in no hurry. Also, it was a very, very busy time—building the instrument, and then fielding it, and then getting ready for the next campaign. These came every two years. It was really busy. So finding the time to write up and finish—

ZIERLER: Did you feel the budgetary crunch at the end of the Cold War?

WENNBERG: No. That was another thing that is amazing about Jim Anderson's group, was it was incredibly wealthy. The idea that you have full-time engineering staff in a university research setting, and not just one. Jim Oliver, who worked with me on the ER-2 project—he's a mechanical engineer, and he had previously invented the spool on the camera that was the first military spy satellite. There was no way to take imagery and transmit it to Earth. This program he worked on was to take pictures of the Earth, with a camera on film, and then to drop the film and have someone catch it, on an airplane. He built the little thing that would spool up the film and throw it overboard at the right time. He was the one who I worked with most on the mechanical side. Then there was an electrical engineer, Joe Demuse, and a software person, Norton Allen. So, it was deeply technically capable. I then end up hiring a lot of these people as contractors to work with me when I move here, just over the internet kind of thing. Because you could never replicate this today, I don't think. It was just this time of plenty.

On the Importance of Atmospheric Radicals

ZIERLER: When you finally got around to writing the thesis, what were your conclusions? What was the finding of the experiment?

WENNBERG: The finding of the experiment was, the major conclusions were that the odd-hydrogen radicals play an enormously important role, so, OH and HO₂ played an incredibly important role in controlling ozone chemistry in the lower stratosphere.

ZIERLER: Controlling means what, in this context?

WENNBERG: In this context, it means determining how fast or how short the lifetime of ozone is. Then, the second and perhaps probably the most important part was to explain how changes in NOx changed this chemistry. So, not just what the chemistry is but the tendency, with respect to one of the major perturbations, NOx. So that you can understand, for example, why does ozone change after volcanic eruptions and other things that alter the NOx chemistry and then drive changes in the other free radicals. So, my thesis, the last chapter is a paper that appeared in Science that essentially explains how NOx controls the chemistry of the lower stratosphere, through these interactions.

ZIERLER: You mentioned the Montreal Protocol has this assessment mechanism that goes well beyond the actual summit.

WENNBERG: Correct.

ZIERLER: Do these findings play into the assessment?

WENNBERG: Yes.

ZIERLER: First, let's just talk a little bit—what does the assessment protocol look like?

WENNBERG: Just like with the IPCC, most of the assessment is based on models, whereby you are asking, "What does a future look like under scenario A, B, or C?" What we learned over the years was that a model divorced from observations would often have real, real problems, in the sense that—often the way people who don't know anything approach uncertainty estimation in the context of, say, atmospheric simulations or models, is to build your model and then ask, "I have all of these things that I am putting into my model, all these terms, and I'm going to try to figure out what the uncertainty in each of them is, and then I'll do a Monte Carlo estimate"—or some other way of error assessment. What we learned over the years with respect to stratospheric chemistry, and I think it is just generally true in the Earth sciences, is that it's the incompleteness of the model which often is the biggest problem. And the way to test for completeness is actually to measure stuff. The way my thesis and other people, all the work we were doing at the time, really feeds into this, is that we identify deficiencies in the photochemical models, and then propose solutions for where those deficiencies likely come from. That then leads to a lot of laboratory work. As a result of all those steps, you end up with a model which not just can explain what today looks like, but that has you have some confidence that you can understand what tomorrow is going to be. I don't know if that's a long way of saying that we work very closely with people who were doing atmospheric simulations and the data feed into—and it's not just like you throw the data over the transom here, and someone looks at your idea. It's very much of a collaborative environment, where you puzzle over things you're seeing, try to understand why.

ZIERLER: Does the Montreal Protocol have a COP kind of framework, where there is an annual meeting to assess what is going on?

WENNBERG: Sort of, yes.

ZIERLER: Are you there? Are you presenting?

WENNBERG: No. I'm not. The big assessments are done every three or four years, just like with the IPCC assessments. And, yes, I'm involved in those every-four-year kind of things, where you're helping to write and review.

ZIERLER: To go back to the question, do the findings from your graduate work—how are they getting plugged in to know what direction, for better or worse, the ozone hole is going?

WENNBERG: Effectively the way this works is just like in the IPCC today, there are a set of standard scenarios that are run in assessment models. The way this work all feeds into that is those models got better and better and better, and they got better and better and better because of the work that we were doing, in observational—the ozone hole didn't exist in any of these simulations. It was only the fact that it was observationally discovered, and then measurements were obtained the understand the mechanism, and then those were able to be quantified and put into these models. So, that's really sort of this we call it the three-legged stool that you have, of observations, laboratory measurements, and simulation science. You're working in a group that is a very collegial environment. There's obviously competition—who's going to figure out stuff first—

ZIERLER: But there's a common problem.

WENNBERG: —there's a common problem, and there's a set of tools that are needed that generally no one group has the capability of being best at, as it were.

ZIERLER: You mentioned the models are getting better. Is it because of computation or computer software programs? Is that a factor here?

WENNBERG: Not really.

ZIERLER: So, this is pretty low-tech in that regard.

WENNBERG: It's pretty low-tech. This was like, "Oh my god, there's this reaction we didn't know about." [laughs] "If we put that in, now all of a sudden stuff we could not figure out why—"

ZIERLER: A chemist from 100 years ago could get read up pretty quickly on this.

WENNBERG: Exactly. The stratosphere is a pretty simple place from a dynamics point of view that's really quite distinct from the troposphere, where the coupling of the dynamics, and say, cloud formation, is a much more challenging problem.

ZIERLER: Are satellites or radar imagery a factor in the assessment as well? Is there an outside-looking-in perspective?

WENNBERG: The remote sensing does play a role, but ground-based. The golden age of stratospheric observations from space comes just after the discovery of the ozone hole.

ZIERLER: The infrastructure is not ready out of the box?

WENNBERG: Not ready. And a lot of this is being done at JPL, by the way.

ZIERLER: Right, that's what I was going to say.

WENNBERG: And the big, huge buses that NASA was organizing for science observations, they come just a little bit too late to have discovered the ozone hole and the chemistry associated with it. But there were—so, the microwave limb sounder is a very famous instrument that was built at JPL that made large contributions to ozone chemistry and science, because they could observe, now, the global context. But to first order, those observations come after things had been mostly worked out. But there were ground-based equivalents of those, so looking at the rotational transitions that had been used in the late 1980s from the ground, to look at halogen chemistry, for example, from Antarctica, from the surface. So, there was remote sensing, but it was not from space.

ZIERLER: When you defend, and then you decide to stick around, are you not thinking that you're going to pursue an academic career? Like, you leave Harvard, you do a postdoc, then you become a professor somewhere; that's not the path you were on?

WENNBERG: No.

ZIERLER: You were just hanging out, having a good time. Why leave?

WENNBERG: Why leave?

ZIERLER: Wife doesn't want to go. It's good in the group. Funding is good.

WENNBERG: Yeah.

ZIERLER: Did you consider that a postdoc, or was this sort of like going back to a staff position?

WENNBERG: Typically we think about a postdoc as that, right? You get your thesis done, then you go somewhere else and learn something new. This was not really that. This was mostly just hanging out. I didn't have strong professional ambitions. I was really enjoying the science, and doing the science. That's the type of position where you only realize later just the noise of being a faculty member, compared to that sort of ability to just—do science. I was really just having a good time.

ZIERLER: Administratively, it's still Jim's group?

WENNBERG: Yep. I just had my own office now. [laughs]

ZIERLER: Moving up in the world!

WENNBERG: Yeah, moving up!

Tropospheric Chemistry and Interest from Caltech

ZIERLER: What was the new project?

WENNBERG: The new project was starting to work on tropospheric chemistry, and start to try to figure out how to measure things that had not been measured yet. But it all gets interrupted by Caltech, actually.

ZIERLER: Chronologically, this is now 1994, 1995?

WENNBERG: 1995, yes. I'm trying to remember the first time—I'm at a NASA meeting in San Diego—a NASA-organized meeting in San Diego, talking about stratospheric chemistry stuff, and I get an email from Professor Yuk Yung here.

ZIERLER: Oh, wow.

WENNBERG: It's a funny email. It says, "Dear Paul, my name is Yuk Yung, and I'm an atmospheric chemist." So it was like one of these—"If you're ever in Southern California, we would love to have you come by and give a talk." "Dear Professor Yung, I'm in Southern California, and I have the day off tomorrow."

ZIERLER: "Let's hang out."

WENNBERG: He invites me up. So, I come up to Pasadena, and I meet all these ‘weird' people—Tom Tombrello, Steve Koonin, people I didn't know. And they don't know any atmospheric chemistry. Gerry Wasserburg—geochemistry. So it turned out that it was like the classic Caltech that's sniffing around. I had just written another Science paper on upper tropospheric chemistry, I come up for a visit, and then a couple weeks later, I get a faculty offer.

ZIERLER: You didn't mention Seinfeld or Flagan. Where are they in this?

WENNBERG: So, John, for sure. I don't remember if Rick was there, that I remember. I don't remember meeting Rick. But yes, John was one of them. But he had been—I knew John [laughs]. But it was all these other weirdos. So it had turned out that—weirdos in the sense of, "Why are you interested?" It turned out that Ed Stolper and John Seinfeld—so John was the chair of Engineering—

ZIERLER: And Ed was division chair of GPS.

WENNBERG: Of GPS. And they had decided they were going to have a new program. Global Environmental Science, they called it.

ZIERLER: Supported by the Lindes? Are they part of the equation at this point?

WENNBERG: No, they're not part of it at all. But it was sort of an Institute-wide thing, sort of classically top down, compared to the normal way. As a result, they had all these people on the search committee—it wasn't really a search committee—who didn't know anything about atmospheric chemistry or atmospheric science. They just knew that I was somebody that maybe they would look at.

ZIERLER: The Science paper, did that sort of put your name on the map?

WENNBERG: I'm assuming so? But, I don't know. So, I come up here, and I gave a talk, and I met—you meet with these people, and you're like, "Why am I meeting with you?" [laughs] Because I had no idea that this was part of a search process. But it was fun. I was enjoying jousting with these people.

ZIERLER: It was purely a schmooze, or you gave a job talk?

WENNBERG: I gave a talk, but a seminar. I didn't know it was a job talk; see, that's what was funny.

ZIERLER: Now where you are, do you know that that's what a job talk at Caltech is?

WENNBERG: At the time, it was. I think there's real challenges and real problems with that approach, because you end up—it's the good boy network. So I'm sure it was John Seinfeld calls up Jim Anderson and says, "Who have you got?" I think we now know that this is not necessarily the best way to hire people. I just happened to get lucky, and they invited me here to give a talk, and I got a faculty offer.

ZIERLER: Do you remember what you talked about?

WENNBERG: I talked about stratospheric ozone. Then I came for a recruiting visit. My wife Cheryl and I—my wife was pregnant—we were pregnant with our first daughter, Janna, and we came here for a visit, and really hated it. So, we said, "No."

ZIERLER: How was the smog at that point? Had the catalytic converters done their job?

WENNBERG: Yeah, it was a lot better. But socially, this was a weird time, especially for Earth sciences. The geochemists here—so, geochemistry at Caltech is famous.

ZIERLER: Yes.

WENNBERG: They had three or four senior, senior—I mean, like really senior—faculty, who were at each other's throats. And so there had been an attempt to hire here for a long time, and they couldn't hire anybody, because they could never agree on the quality, because each of them was torpedoing the others.

ZIERLER: Just because it was your guy, so—?

WENNBERG: Yeah. So, when we come to visit, this didn't feel like the thriving fun place that—and again, I didn't have ambitions towards this. It's not like, "Oh my god, I have to go be a professor at Caltech." It was really like, "I'm not aspiring to this. Thank you for offering." The visit—my wife was pregnant, as I said, so not a great time for making decisions to really reorganize your whole life.

ZIERLER: And she's probably not any more enthused about leaving Cambridge than she ever was.

WENNBERG: No, she's got all her friends – her support community. When you're starting a family. And so, we just go back. Then, two years later, they offered me tenure to come. Cheryl was like, "Those bastards!" [laughs]

ZIERLER: [laughs]

WENNBERG: But I said, "Oh, come on, Cheryl. Let's go try it out! There's no risk. If we go and we don't like it in a couple years, we'll find something else to do."

ZIERLER: Was it that same line that you filled, that was open for the two years?

WENNBERG: Yeah.

ZIERLER: That's the Caltech way. They'll leave something open rather than fill it with someone they don't like.

WENNBERG: Yeah. And we came, and of course the upshot is that Cheryl loves it here. She found a good job that she really enjoyed —and we would go back to Cambridge, and she'd say, "I can't believe I ever thought we should stay here." [laughs] So that's how we ended up here. A few years later. That was 1998.

ZIERLER: To go back to the postdoc research, you mentioned there's tropospheric chemistry and there's stratospheric chemistry.

WENNBERG: Yeah.

ZIERLER: You were doing both.

WENNBERG: Yeah.

ZIERLER: Do they go together? Is it a hand-in-glove kind of thing?

WENNBERG: Some of the molecules are the same, of interest, and the techniques to measure stuff are the same. But the chemistry itself is really diversely different. I think what I felt like is after the work we had done in the 1980s and 1990s, the stratosphere had a sense—there were still some puzzles, but it didn't seem like this was a place to start your career, in the sense that there were big outstanding problems. That said, when I came here, I did start with some stratospheric stuff, because, well, it was a way to get your foot in the door and fund your lab and start to build stuff. The problems weren't uninteresting, but you didn't feel like it was going to be a career of continuing to work on stratospheric chemistry. The troposphere, on the other hand, was clearly undersampled and there were a lot of first-order questions that I thought I could help answer. That's how I ended up more focusing on the tropospheric chemistry.

ZIERLER: From the thesis to the postdoc, what was a continuation or an expansion, and what was brand-new?

WENNBERG: Essentially we were just trying to fly the plane in other places. That was the big thing.

ZIERLER: So it really was an extension of a really long extended thesis.

WENNBERG: It was. It was being able to now—and then thinking about, what were we missing, and how to build machines that could fill in some of those holes that you needed to fill, to do tropospheric chemistry research. I tried to get some funding to build some new things, and never really got started on it before I moved here.

ZIERLER: Are you following the policy processes by which the ozone hole is shrinking? Are you following how your research is getting plugged into new regulations?

WENNBERG: Yeah. And as we learned more and more, those got tightened and tightened. As we realized—people think the Montreal Protocol banned the CFCs, but it didn't. It was really just an attempt to keep the production from going up. There were all these subsequent amendments that lead to the stricter and stricter controls on releasing long-lived halogens. And it's still going on today. We're still trying to figure out how fast we can shut down some of the perfluorinated compound use.

ZIERLER: You mean in other countries?

WENNBERG: Yeah. And I still follow this, mostly out of curiosity, but also because people ask me about it, so I feel like given my background, I should have an informed opinion.

ZIERLER: By flying the plane in new places, what new things did you learn?

WENNBERG: You start from the top down. So, we fly in the upper troposphere, and there's just a lot more OH up there than you would have expected based on photochemical models, so we spent a lot of time trying to figure out why that was. We still have some ideas, but it's clearly related to transport of oxygenate organic compounds from the surface that lead to really different chemistry than was in the models. This led to some of the first work in my laboratory after coming to Caltech. We – Coleen Roehl, a post doc and now staff scientist, Sergey Nizkorodov, now a professor at UC-Irvine and one of my first graduate students, Julie Fry – discovered new photochemistry of peroxides, both organic and inorganic that leads to the production of OH in the upper troposphere. Then more recently I've been working on organic atmospheric chemistry as it relates to air pollution, and trying to understand much more about the biosphere and its interactions with atmospheric chemistry.

So, my group becomes well-known for both its observational side in the atmosphere, but also in the laboratory, figuring out how molecules react. I've had a number of really, really amazing students and postdocs who have carried this work. One of my first graduate students, John Crounse, he has had a career similar in some ways to mine, in that he goes to college and then goes to work in analytical chemistry in a small company, but eventually then comes to Caltech. So, a little bit more senior as a graduate student. Comes to work in my lab, and he's just a—he grew up tinkering, knows how to build stuff, very, very talented. He really is the one who works out a lot of the systematics for how we can use mass spectroscopy for measuring the kind of molecules that we do. So, he and I have just continued. He stayed on and then became basically the guy who keeps my lab going. Again, with somewhat of a two-body problem; his wife is from Southern California. So, he could have gotten a faculty position anywhere, I think, but decides it's fun just to work on science.

ZIERLER: Who reaches out to you the second time from Caltech?

WENNBERG: Michael Hoffmann.

ZIERLER: Oh! Okay.

WENNBERG: Yeah. Mike, who visited Harvard quite a bit, because he had students and others who had been there. He came to see me at the lab, and said, "So!" Anyway, yeah.

ZIERLER: No job talk this time? They just offered it to you? With tenure!

WENNBERG: They were desperate.

ZIERLER: I guess so!

WENNBERG: Yeah. As I say, when students ask me for career advice on academia, I say, "You can't listen to me."

ZIERLER: [laughs] Except for the fact that you were so not careerist, and focused on the science. Clearly that was good for your career.

WENNBERG: But again, I think luck had so much to do with why I'm here today. I don't know if that's still the case. I've been involved in a number of recent faculty searches here, and it's incredible, the talent out there, and the amount of—I guess I describe myself as being sort of just pretty much just focused on something because I find it is interesting. I think today, a lot of—people are just much more thoughtful about how they organize themselves, I think. If they're aspiring to a faculty position, they have an approach which is just much more—because it has to be—much more organized around the search, and how do you land a position when the competition is so hard right now. I think when I graduated from college, when I got my PhD, I'm going to guess that probably a large fraction of the PhDs could go on to academia. Just the numbers worked out that way. Of course, then, if you're from Harvard or MIT or Caltech, the chances were even higher. And it's just not the case anymore. It's much more competitive. Our students don't all necessarily aspire to this, either. They really see—if you wanted to say, "I really want to make a profound difference in environmental science," is academia really the route to that today? I think for a lot of our students, they say no, that there are things they can do, science-adjacent, or in the sciences but that are not associated with higher ed.

Environmental Science and Environmentalism

ZIERLER: The last topic I want to touch on today, that I'll tie back to the beginning—we really haven't covered your environmentalist sensibilities in all of this. Growing up around nature in Vermont clearly is formative. But what about just being part of that political-scientific process, of understanding this massive environmental problem? In other words, by the time you became a faculty member at Caltech, how well solidified were your ideas about environmentalism, science, where to balance them, where to keep them separate? Where were your thoughts at that point, by the time you joined the faculty here?

WENNBERG: I don't know if I still feel this way today, but at the time, and I still think maybe even today, I think the role of the natural scientist in putting forward and learning stuff about how we interact with the environment—that's separate from the politics—is sort of the way I've organized what I do. So, I'm interested in the policies and the politics, because I follow them, and because it's interesting. Because in the end, my goal, of course, is to provide unassailable science towards practical and good decision-making. And I'm motivated that we want to leave the planet better off, but how that translation occurs once you basically tell people what you think a current trajectory is looking like—you understand what is going to happen based on your science—the response to that, the political response, the policy response, has been something I haven't really engaged in. Other than as a consultant on the science.

ZIERLER: It was never hard for you, then, to separate those things out, because you didn't really have a forum to force you to make those decisions.

WENNBERG: That's right. I'm just not part of that advocacy in that way.

ZIERLER: And that's very Caltech.

WENNBERG: Maybe so. I think people leave here if they want that type of impact. Arie Haagen-Smit, who famously figured out why we have smog in Los Angeles, once he realizes that he needs to engage in the policy and the politics, he does so by resigning. I think I would feel the same way—that if I decided I want to have a career in policy or politics, then I would stop doing what I'm doing here, and do something else.

ZIERLER: In your private life, like if you're a member of Greenpeace or anything like that, that's just two different worlds, as far as you're concerned.

WENNBERG: That's right. Two different worlds.

ZIERLER: And it's better that way.

WENNBERG: I don't know. [laughs] It works for me.

ZIERLER: In the sense that you never want anybody to second-guess your science because there are—because in the field—I mean, atmospheric science, there's—

WENNBERG: I used to agree with you; I don't think they're separable anymore. This idea that the public will respond to you and your science because they don't see you as an advocate; the way things have gone recently, it just feels like everything has become politicized. You can see that in just the discussions around global warming. If you say something—if I put out a paper about—anything—the political angle conversation just overwhelms almost as fast as you can even try to make sure that the press release is right. So, I don't know. It's a little bit depressing about how we can't just agree on the facts anymore, and that attempts to bring facts forward often are not nearly as successful as you would hope.

ZIERLER: This is a worsening trajectory that you've witnessed.

WENNBERG: Oh, yeah. Definitely. The fundamental—I think when I started in this area, and if you said you're working on environmental chemistry or you're working on this problem, the biggest response you would get would be curiosity. "Oh, that's interesting. What have you been learning? What does it tell us about the system works?"

ZIERLER: The assumption is your motivation is curiosity.

WENNBERG: Curiosity, yeah. And they're curious, and people were curious. Now, it feels like, "So what's the political angle around this?"

ZIERLER: That's unfortunate.

WENNBERG: Yeah. I think so. I think it slows down progress.

ZIERLER: On that happy note—

WENNBERG: [laughs]

ZIERLER: —we'll pick up next time when you actually start with the faculty at Caltech.

WENNBERG: Okay! That sounds like fun.

[End of Recording]

ZIERLER: This is David Zierler, Director of the Caltech Heritage Project. It is Friday, March 10th, 2023. I am delighted to be back with Professor Paul Wennberg. Paul, once again, thank you so much for having me.

WENNBERG: My pleasure, David!

ZIERLER: Today, we're going to pick up right at the moment you join the Caltech faculty. We covered the back story to how that all came together. Just an administrative question to start—you came in as an associate professor, obviously when Caltech still had that designation. Was the idea that you had been a postdoc for long enough, but you were too young to be a full professor, and that associate bridged the difference?

WENNBERG: At the time, it wouldn't have been appropriate for me to be a full professor.

ZIERLER: Just by age and publications?

WENNBERG: By age and publications and everything else, yeah.

ZIERLER: But you did come in tenured?

WENNBERG: I did, yeah. And today, there still are people who have associate professor titles here on campus. If they've come in at the senior but not quite tenured—

ZIERLER: Ah!

WENNBERG: —we'll often—

ZIERLER: Oh, so it hasn't been fully abolished.

WENNBERG: No. They will still get an associate professor, and then come up for tenure and full professorship maybe a year or two later.

ZIERLER: I know EAS was sort of at the forefront of, "Let's get rid of the associate designation institutionally." Is your experience, do you think, a good case study in why that level is valuable?

WENNBERG: I was here when the Institute got rid of that title. I think there was this broad sense that it was arbitrarily applied, that when somebody was promoted to full had much more to do with—

ZIERLER: Personalities?

WENNBERG: —personalities, but maybe external offers, or other things. I guess in Humanities, they have a real definition of what it was: you publish one book; you get tenure. You publish your second book; you'd get full. So, there's something like this, right? I don't know the full story.

ZIERLER: Humanities is more quantitative! [laughs]

WENNBERG: But in the sciences, it tended to be something that was I think much more arbitrary in terms of when it happened, and the process by which it happened. It also was a lot of work. You didn't get a big raise when you got promoted to full. So, at some point it became, why are we wasting our time on this? It's nice to celebrate your faculty when they're really successful, but I think we do that pretty well at Caltech without having to go through this gate that requires a lot of work by external people to write letters and so forth and so on.

The Caltech Legacy in Atmospheric Research

ZIERLER: In what ways, even before joining the faculty—how did you think about Caltech institutionally, in the history of atmospheric chemistry? Was there a program that you associated with? Was it individuals? Was it impact? How did you think about atmospheric chemistry at Caltech even before you got here?

WENNBERG: Caltech was one of the epicenters of the field, in terms of intellectually driving things forward. It was in Engineering, it was in Chemical Engineering. But I came from a different community. I came through physical chemistry. So, Caltech was, I would say, somewhat less known in that particular community. That tended to revolve around other places. Yet, everyone, anyone, who was doing atmospheric chemistry knew that Caltech was like the place. Because you remember, right, in the 1950s, Arie Haagen-Smit was really the revolutionary figure to have brought huge amounts of knowledge to air pollution chemistry. Then, he was succeeded by a lot of very impactful atmospheric chemists, and people who came to Caltech without being atmospheric chemists became atmospheric chemists. Arie Haagen-Smit was a flavor chemist. John Seinfeld was a mathematician. I don't mean that badly, but that's his focus, was—and he decided building models of the atmosphere was a useful tool to promote what he was doing. So, everyone knows him as an atmospheric chemist, but he wasn't trained as one. Glen Cass, who became the chair of the program at Georgia Tech, he was here for many, many years. There's just a whole group of environmental chemists who played an enormous role in putting Caltech on the map, through their impact.

ZIERLER: In the way that Caltech's smallness imposes—I don't know if "imposes" is the right word, but there's an expectation that the professor is going to be a real leader in that area, like a real worldwide leader. In what ways institutionally did Caltech support you toward that goal?

WENNBERG: One of the things that was perhaps somewhat surprising when I tell people is how naïve I was.

ZIERLER: You weren't a faculty member before.

WENNBERG: I wasn't a faculty member. I also hadn't really aspired to be one. I hadn't really talked to people about what that was, other than the models that you know around you. So, this sort of whole like, "What do you ask for? What do you get? How do you use it?"—to me, it was just like, "Oh my god, it's raining resources on me!"

ZIERLER: And you did feel that?

WENNBERG: Oh, yeah! Huge!

ZIERLER: Coming from where? The division chair, the provost?

WENNBERG: All of it. This Institute I think is remarkable in its ability to garner funds to support its starting faculty to make sure that they're successful. Ed Stolper was the division chair, and one of the things he told me, which later a lot of my colleagues elsewhere were just incredulous about, was that, "You don't need to ask for everything now. As you get started and you learn, just come back and we'll give you more."

ZIERLER: Because you might switch course, and you might not need what you asked for.

WENNBERG: Right. But if you talk to anyone else at any other institution, they'd say, "Oh, never believe that. You have to get what you're going to get at the beginning!"

ZIERLER: You have one opportunity.

WENNBERG: You've got one shot, right? [laughs]

ZIERLER: And you read the writing on the wall, or this was articulated to you?

WENNBERG: He directly told me.

ZIERLER: That's great.

WENNBERG: I never did have to go back, in the same way, so maybe—I don't know what the answer would have been. But I do know that we are very good at supporting our junior faculty getting off the ground. Now, I was tenured, but for all practical purposes, I was a new professor and had all the same problems—raising money, getting a lab started.

ZIERLER: What was the game plan for the lab? And we will superimpose that on the research that you were focused on circa 2001, 2002. What is the research and how is the lab going to be responsive as you are building it?

WENNBERG: Again, I feel like I didn't have a particularly strong strategy. This was very different than what you see today, because you read the applications for new professors here, and they all have a very clear plan. Partly maybe because I didn't apply!

ZIERLER: This is in keeping of your very chill attitude when you were a postdoc. It's working; you're having a good time.

WENNBERG: Maybe so, maybe so. But it's also I never had to write an application, and so I never had to—

ZIERLER: —articulate—

WENNBERG: —to myself, either! [laughs] It's not just articulate outwards. It's also, like, "Okay, Paul, you're going to be a new professor. What's your plan?" I sort of developed that while doing it. I realized what tools I would need, and some of the new things I wanted to do. I wanted to get into mass spectroscopy. I wanted to continue to work on atmospheric compositions. And I thought the excitement—my own, and my students—around field work was an important organizing principle, and one that there wasn't a terrible amount here on campus. So, we didn't have an airborne program in atmospheric chemistry, really, until I came. Then John Seinfeld later, with Rick Flagan, started their work with the Navy.

ZIERLER: Office of Naval Research?

WENNBERG: Yeah. But I really came here with this idea, and I worked—actually, in collaboration with Rick Flagan and Fred Eisele from the National Center for Atmospheric Research —to develop a really fun airborne sensor for the NASA U-2 airplane, the one I had worked on during my thesis. It was again a stratospheric experiment to look at both the particulate and the gas phase acids in the stratosphere, so it was very much completely linear in what I had been doing. I think a lot of faculty are that way. You want to do something that you know is going to work and will have impact, and then you can risk doing something else, something new. This was sort of my more like in the heart of what I'm able to do and my interests in doing things. I was also able to attract two really good postdocs to pull this off – Karena McKinney, now a professor at Colby College and Suresh Dhaniyala, now a professor at Clarkson University.

ZIERLER: We covered this in our first talk, but in the chronology, was there ever back and forth about you actually having a CCE affiliation, or just being a chemist, being in CCE?

WENNBERG: My understanding—and I think it's still the case today—is that you can be associated with two divisions only. Because my hiring was done through two already, there was not such an option.

ZIERLER: Did that make the most sense to you?

WENNBERG: I don't know!

ZIERLER: [laughs]

WENNBERG: In retrospect, I've had so many chemists working in my group.

ZIERLER: Yeah, but the other way of thinking about it is that the administrative walls here are so low anyway, like, who cares.

WENNBERG: Correct. Right. As soon as I got here, the chemists said, "Come give a talk at the Wednesday pitch talk, for the first-years." So, it was incredibly welcoming, and the students found me. But if you go to the CCE faculty page, you won't find me there. That does, I think, maybe reduce my visibility among that subset of students applying for graduate school. They have to actually know that they're interested in atmospheric chemistry and know about my lab here.

ZIERLER: Were there advances in the technology of mass spectroscopy around the early 2000s that compelled you to want to jump in more fully? It's an old technology but things are always improving.

WENNBERG: Yes. There's a technique that's called chemical ionization mass spec developed by my collaborator Fred Eisele, that is used in atmospheric chemistry that is really quite different from the sort of SIMS that you hear about, in that you can do selected ion clustering. So you can make an anion -

ZIERLER: What is that, anion?

WENNBERG: We use CF3O- and the systematic kinetics of that anion had just been worked out at a Boulder lab.

ZIERLER: Boulder meaning NOAA?

WENNBERG: At NOAA, yes, by Greg Huey. And it was a particular anion for studying stratospheric gases, because it clusters with almost all the things that you're interested in studying. The nice thing about it is, it doesn't cluster with oxygen, nitrogen, water, very much. And so you don't have to do any chromatography. You don't need to do anything other than send this ion through ambient air, and then see what clusters it forms.

ZIERLER: What does that look like? What is the experiment when you're sending it through this ambient air?

WENNBERG: In our case what we had was what's called a virtual impactor. This is an inlet – designed by Suresh Dhaniyala and Rick Flagan - that allows you to either selectively collect the particles by excluding the gas and then you can evaporate them and measure what's in them. Or, you can do the opposite. You can selectively, by inertia, remove the particles, and just measure the gas. The idea of the sensor was that we would measure something like, say, nitric acid, present in both the gas phase, and independently in the aerosol phase. The idea, then, is you bring this air sample in, warm it up, and then expose it to CF3O- clustering. Then you have a mass spec. The mass filter was just not particularly innovative. It was a quadrupole mass spec. We now use time-of-flight because it's a much better mass filter for what we're interested in. But, we used a quadrupole. The only unique aspect of that was the environmental aspects. So, you're flying on a single piloted airplane, with minimal weight allowed for the instrument pod. So we had to build something—

ZIERLER: Light.

WENNBERG: —light.

ZIERLER: Like carbon fiber and aluminum light?

WENNBERG: Not so much, but aluminum for sure. The other thing you had to do was to develop it to be an automated sensor. So it's pretty complicated operations, that we were doing, where we would be doing this gas and aerosol phase.

ZIERLER: Automated—is there any AI going on in this?

WENNBERG: Oh, no, no, no. This is all script-based. But there's some, like, "Don't turn on until the pressure is lower than this," and these kinds of things. Gates. But we also had to—so, we needed a lot of nitrogen for this instrument. Because you are bringing the particles into a stream of air, so you need a clean stream of air.

ZIERLER: Clean means what? It's just what you're catching?

WENNBERG: Whatever you're catching in air, you want to be able to take these particles, evaporate them, in something that is devoid of any of the molecules you are studying. So that turns out, it's nitrogen. But there's no way to carry the amount of nitrogen that we needed in a high pressure gas cylinder, because the weight would be excessive. So we built a nitrogen cryopump, a liquid nitrogen cryopump. This pump—it serves two purposes. It backs the turbo pumps. On the mass spec, you have high-vacuum pumps. They need a pump to collect all the stuff that they're pumping. Typically at the time, that would have been a rotary pump or some very heavy mechanical pump. But what we did was we had instead a cryopump, which is a basically sorbent that's cooled to liquid nitrogen temperatures. It absorbs nitrogen, oxygen, very, very well. Helium, not so much, but there's not enough helium in the atmospheric air to matter. So, this thing would back the turbo pumps, meaning create a low pressure for them to exhaust into. At the same time as the boil-off from the liquid nitrogen provided all of the nitrogen that we needed for the sensor, during the flight. So it was a very efficient mass way to get both the pumping we needed and the gas. That was perhaps the most innovative part.

ZIERLER: How long are flight times?

WENNBERG: Typically—as long as ten hours, but usually six to eight.

ZIERLER: These are Air Force pilots?

WENNBERG: Yes, Air Force pilots. Driving a NASA plane, though. The plane is NASA.

ZIERLER: This is NASA funding.

WENNBERG: Yeah. And actually the pilots work for NASA, but they are ex-military.

ZIERLER: Oh, they're ex-military civilian NASA employees.

WENNBERG: Yes.

ZIERLER: Oh, that's a good gig! [laughs]

WENNBERG: Yeah. Retired from the military, worked for NASA.

ZIERLER: Do you get to hang out with these guys? Is there an interpersonal kind of aspect?

WENNBERG: Oh, yeah. They're generally pretty—they're around, and generally quite dismissive of the scientists. [laughs]

ZIERLER: [laughs]

WENNBERG: But, it was great. You know, one of them actually worked—had done a degree at GALCIT.

ZIERLER: Oh, wow.

WENNBERG: Yeah. I forget his name, the pilot.

ZIERLER: Where do you take off? One airfield, or you're all over the globe?

WENNBERG: All over. Yeah. The experiment that we did, the first one I did, in 2000 when I got here—so this was just a year and a half after getting started—was in Kiruna, Sweden, to look at polar ozone chemistry in the Arctic. You fly there on military transport, and the plane of course just has a single pilot. Goes from—at the time, the plane was—I think it was still up in the Bay Area, at Moffett Field, but it may have already been down—because now, the U-2s – ER-2s - are right out here in Lancaster, Palmdale.

ZIERLER: One area is Arctic. What about Antarctic, I assume, as well?

WENNBERG: Yeah. I had done work in flying the airplane out of New Zealand, previously. But basically by the time—I think we talked about this—by the time I got to Caltech, it really felt like the stratosphere was pretty close to being a solved chemical problem.

Global Flights to Understand Ozone Chemistry

ZIERLER: Meaning what? What were the questions that were resolved?

WENNBERG: We understood the basic dynamics of ozone chemistry. We knew a lot more about what the source gases were to that, what things were being emitted in the troposphere that led to the chemistry up there being perturbed.

ZIERLER: These are both human and natural processes?

WENNBERG: Yes. And the aerosol chemistry became much, much clearer. The questions we did get involved in thinking about by the time I was coming to Caltech were about somewhat unique things about, how do you lose nitric acid and water from the stratosphere? This was an open mystery as to why—in order to build particles that are big enough to sediment—so if you want to gather up some nitric acid and water, and have those particles fall fast enough that they make it out of the stratosphere, first of all, it has to be cold. But second of all, you need to—

ZIERLER: Cold because it just drops?

WENNBERG: Because you only condense them on the particles when it's cold enough. Then the second thing was, you need that not all the particles nucleate to form ice. So these are not water ices, but they're nitric acid trihydrate, is a crystalline form of three nitric acids and one water. The other way around—three waters, one nitric acid. And if all the particles became such ice nuclei, then because there's so many of them, no individual gets to be big enough to fall. So what you need is for what's called selective nucleation. In other words, you need just a few of them to become ices. Then when that happens, they can suck up the nitric acid from all the other liquid particles. So you basically have a little lower vapor pressure pump that's falling through and collecting all nitric acid. So we knew that the nitric acid was going away, that it was being lost to the troposphere, the lower atmosphere, but this nucleation was a real mystery. That was one of the things we contributed to, during that first campaign, when I was at Caltech, which was we came to realize that you could explain this based on what are called Lee waves.

ZIERLER: L-E-E?

WENNBERG: Yes. You will see these if you go to the Rockies, and you—on the eastern side of the Rockies, what you'll notice is a set of striated clouds, almost in a line, and then there's an echo of them downwind, so to the east. These are what are called Lee waves. The air moving over the mountains is lofted, and it forms a cloud, because as it's moving upwards, it cools. Then you have a gravity wave, so these things just then bounce. The air is just moving upwards and downwards, by gravitational force, so expansion and then compression. As it moves upwards it cools and forms a cloud; then, it moves downwards, and it warms, and the cloud goes away. Then as it moves upwards again, it forms a cloud, and so on. Then it dissipates. So, when you go and look at the mountains, you'll notice these things called Lee waves. In the stratosphere, these also happen. In the lofting part, you can nucleate the particles so they get really cold, and then they form the ices. But they do so in a very narrow, thin layer. Then as long as it's cold enough that the compressional aspect does not get back to the melting point, which it really doesn't for a lot of these, those particles can remain. So you end up with this very thin layer, where you nucleate all the particles. But remember, I said you can't do that, because you won't be able to—

ZIERLER: Right.

WENNBERG: But because it's so thin, then they gravitationally settle over days, out of this thin little layer, until they get just below it, where now, there's plenty of nitric acid and water, and then they can start to grow. Suresh, Karena, and I call it a little salt shaker. You have this small, thin layer of particles, ice particles, that then sediment, and once they sediment out of this thin layer, they grow, and lead to the nitric acid loss. I think this is pretty much now accepted as the mechanism. It's also important now in the troposphere, we realize. In regions where you don't have ice nuclei, this is a way in which you can dehydrate the upper troposphere as well. That these Lee waves are an important part of the story.

ZIERLER: How much of this analysis is happening on site and how much are you bringing back to the lab?

WENNBERG: This is after we got back to the lab.

ZIERLER: What does the thing look like that you bring back to analyze?

WENNBERG: Okay, so one of the things that I brought back—and I think this is important—is I brought back pictures. We always like to think, you just go on the computer these days and you learn stuff—and it's true—but there's still a really important component of going to the field.

ZIERLER: Visualizing.

WENNBERG: So, you go to Kiruna, and you're on the eastern side in the mountains that project across the peninsula up to the Arctic.

ZIERLER: This is way at the top of Sweden?

WENNBERG: Yeah. And what you see are beautiful stratospheric noctilucent clouds.

ZIERLER: What do they look like?

WENNBERG: They're purples and reds. They're like the inside of a clam shell. You know those colors?

ZIERLER: Iridescent.

WENNBERG: Yeah. They are very thin, and you see them—just like a Lee wave, you see these little thin things. And everyone is going, "Wow, that's really cool." And I'm going, "Yeah, that's really cool." And then you get back, and you're like, "Ah!"

ZIERLER: "I wish I had a photo."

WENNBERG: No, you had a photo! Now, I know! And now I'm like, oh, that's how this is happening! You're actually seeing these Lee waves in the field, associated with—at the moment, they're mostly curiosities. You just see them up there, and—

ZIERLER: What's the camera? What are you using to capture these images?

WENNBERG: Oh, people are just taking [laughs]—just a—

ZIERLER: Oh, just a—

WENNBERG: Literally—you didn't have a phone camera at the time, so they didn't have a phone camera.

ZIERLER: But just like little digital cameras?

WENNBERG: Yeah! [laughs] Or, you could just say, "I saw it, and it just triggered an idea." I do think part of being an observational scientist is to open your eyes. Go to the field. And you see things.

ZIERLER: Was it obvious to you when you joined the faculty that NASA would be a big deal in your research agenda, or that happened in real time? You knew?

WENNBERG: Yeah. I had come from a lab that was completely supported by NASA.

ZIERLER: This is separate from JPL.

WENNBERG: Yeah.

ZIERLER: You don't need the institutional JPL connection.

WENNBERG: No, no.

ZIERLER: Is JPL being there helpful to you? Does it make things smoother?

WENNBERG: Well, that brings us to the second part of what I did when I came here.

ZIERLER: Oh, perfect.

WENNBERG: So that's a good lead. So, there were people at JPL doing stratospheric science, and they had a vigorous program in remote sensing, but also in situ stuff. So, I had natural colleagues there, people I knew, some whom I had known before coming here. People like Ross Salawitch and others, who I interacted with at Harvard and other places, they were at the Lab. At the time it was really easy to go to the Lab. Now, it's much harder because of all the security stuff, but at the time you just walked in, pretty much. So I used to go out and hang out up there with some friends. There were a group of four or five of us stratospheric people who were like, "You know, climate change is the thing. How can we contribute? How could we contribute to this science?"

ZIERLER: Were you thinking along those lines yourself at that point?

WENNBERG: Yes. At Harvard, I had taught a course as a TA, sort of introduction to atmospheric science for undergrads who aren't doing atmospheric science. The "Atmospheres for Poets" equivalent. So, I had been exposed—I think we talked about this last time—to global warming science. 1998, when I came here, this was already a big topic, right? It's not like this is coming out of nowhere.

ZIERLER: It's a big topic, but it's a young topic, still.

WENNBERG: But it is young. Well, youngish. Certainly—yeah, in the context of what it is today, that's true. But of course the science goes back to the 19th century—

ZIERLER: I guess I mean young in the sense that there is a zeitgeist in the scientific community. Alarm bells are going off. "We gotta do this."

WENNBERG: Correct. That's exactly right. At the same time that the ozone hole was appearing, everyone was going, "Oh my god, we are really hitting this climate system hard." So then the thought was, "How could I"—and how could we, for my colleagues up there—"use our analytical skills to address important questions in the climate?"

The Origin of the OCO Mission

ZIERLER: Is this the origins of the Total Carbon Column Observing Network?

WENNBERG: Only sort of. This is the origin of the Orbiting Carbon Observatory.

ZIERLER: Are they sequential? Does one lead into the other?

WENNBERG: Yes. Very much. This is another—again, my life as a lucky person—story, because—so, there's a group of us: Bashwar Sen, Chip Miller, Ross Salawitch, Geoff Toon, Yuk Yung was here, and me, who just started chatting. Like, could you measure CO₂, and could you measure it from space? And could you measure it—it's easy to measure CO₂ from space. The question was, could you measure it with sufficient precision?

ZIERLER: Why is it easy to measure from space?

WENNBERG: The reason why it's important is because it absorbs in the infrared, a lot.

ZIERLER: So you just see where there's more infrared? It's as easy as that?

WENNBERG: Right. Or actually less, right?

ZIERLER: Yes, of course. [laughs]

WENNBERG: So, it's eminently observable. It's different to measure it with the precision you would need. Because basically CO₂ is the same everywhere. It's within a few percent of the same amount, or the same mixing ratio as we say, everywhere.

ZIERLER: This is only carbon, or it's all greenhouse gases that are easy? Like methane?

WENNBERG: Yeah, methane as well.

ZIERLER: Same technique, same analysis?

WENNBERG: Yeah.

ZIERLER: Can you differentiate, when you're looking at the models, where's the carbon and where's the methane?

WENNBERG: Yes. The challenge, though, the analytical challenge, was measuring it well enough that you could see really small differences in CO₂ between places where CO₂ was entering the atmosphere and where it was being taken out. Because it's those differences that tell you about the strength of the sources and the sinks. What we were focused on at the time, and still, was trying to understand, how is the natural carbon cycle taking up carbon via the growth of the forests? About half of the CO2 that is emitted into the air by humans goes missing. Half accumulates in the atmosphere, and the other half goes into the land and the ocean. About half of the half, so about a quarter, goes into the land. But it was really not understood why.

ZIERLER: But this is a good thing in terms of natural mitigation?

WENNBERG: Oh, yes. This is an ecosystem service. The CO₂ would be going up twice as fast, if this didn't happen.

ZIERLER: Lucky us.

WENNBERG: Lucky us, right! We wouldn't have two to three ppm increase per year; we'd have four to six. The ocean part is relatively easy to understand. It's just Le Chatelier's principle. It's effectively a physical thing. The amount in the atmosphere is increasing, so the oceans are subsaturated, so they take up CO₂. But the biosphere part was really, and still is, pretty much mysterious. Like, why do the forests grow? Where are they growing? Where are they taking it up? Will that carbon remain sequestered? How will this evolve as the climate system warms? These are all just really unknown questions that we're still working on today.

ZIERLER: If I can ask a strategic and a tactical question, we've already compared the Montreal Protocol with the IPCC. One obvious difference, as you're jumping into this field, is that the ozone hole is not an intractable problem in the sense that we can identify the offending chemical and we can pretty much eliminate it and go on with the way we live our lives. Carbon dioxide, that's like a 100-year proposition. This is going to happen in fits and starts. So, if you can take that observation, does that affect the kind of science, the way you're approaching it? Or does that not really matter, because you're just in a fundamental research mode, and you're learning the dynamics?

WENNBERG: I'd say a little bit of both. That's a wishy-washy answer, but I would say that we were very cognizant of the fact that we were committing the Earth, through this perturbation, to a long period of warmth.

ZIERLER: That's baked in, already, even if we stop tomorrow.

WENNBERG: That's right. However, just like with the CFCs, the chlorofluorocarbons, you're doing this long-term experiment, you don't know the answer; you better quickly try to figure out what the answer looks like, right? Because if you're going to force a large reduction in greenhouse gas emissions, it's going to be incredibly expensive and disruptive. So, you need to have all the evidence that you can muster, to understand what that experiment that we're doing looks like in 100 years. Again, what you are doing is simply asking fundamental questions, yes, but the answer to which is profoundly important. Because you can say it's baked in, and that's true about last year's emissions, but that's not true about next year's right?

ZIERLER: It's true systematically in the sense that we can extrapolate, "It ain't going anywhere next year."

WENNBERG: Right.

ZIERLER: We can try, we can reduce, but it's still happening.

WENNBERG: Yeah. Even so, but again—the emissions of the future are not baked in, and they depend on what we know and what we learn.

ZIERLER: This is the environmentalist in you.

WENNBERG: Yeah!

ZIERLER: "We can do something about this."

WENNBERG: Well, we should first know whether it's important to do it. Because there's only so many things you can do. You only have so much resource. You should focus on the things that you can do and should do. I really feel like at the time, in that year, 2000, I'd say there was a lot of debate about what the future looks like, and what the impact of this ‘experiment' we were doing was going to be. In the face of such uncertainty, the insurance answer would be you do the maximum amount. But the policy answer is you don't do anything. I mean, how insurance markets work in the face of risk, and how policy and governments work, are complete opposites. And so, because this is a commons problem, that the CO₂ emitted in China or in—

ZIERLER: Tragedy of the commons, you mean.

WENNBERG: Yeah. It doesn't matter. This is a—

ZIERLER: It's one shared atmosphere. It's the ultimate global commons.

WENNBERG: Yeah. And it doesn't have to be a tragedy.

ZIERLER: I like this. Right.

WENNBERG: Right? Tragedy is what happens when the value of the common good is zero.

ZIERLER: Although it depends who you ask. There are tragic victims of climate change already.

WENNBERG: Already, yeah. Absolutely.

ZIERLER: Let me ask the tactical question. You've touched on it a little bit. You're obviously coming from the fundamental research perspective, but because the outcome and the motivation of the research is so obviously applied, does that affect your approach? Do you need to learn how to become a different kind of scientist as a result?

WENNBERG: No. [laughs]

ZIERLER: And is the ozone research like the game plan?

WENNBERG: Yeah, exactly. The same—and in fact, it's interesting—the IPCC, which is responsible for periodically assessing the state of climate science—the leadership of, and the structure of, all follows directly from the assessment of ozone science. It's the same people, same approach.

ZIERLER: You mentioned policies, early 2000s. We're talking about Bush, Cheney, Haliburton, Texas oil. Are those boogeymen kind of issues, or do you really feel a clamping down on what otherwise could be an unencumbered, emergency-based research endeavor?

WENNBERG: Hmm, that's interesting. I don't know. I don't really remember. I think we talked about this a little bit last time, which is the joy of—the benefit of the incredibly well-funded federal science program in the United States is that relative to everything else the government does, it's a roundoff error. And so, it generally doesn't provoke attention, and it in general has been fairly bipartisan, the support for the scientific enterprise.

ZIERLER: There are Fox News talking points, but then the actual budget, it doesn't translate.

WENNBERG: Yeah. You talk to congresspeople and their staff, and they're generally excited to learn stuff. Maybe they have to appear a little more partisan in their commentary, but—and it does lead to silliness, like, "Don't call it climate change," or "Don't call it global warming; call it atmospheric science." There's the usual marketing aspects of trying to make sure that you keep your head down. But the program that we proposed to was this new program at NASA called the ESSP program—the Earth System Science Pathfinder. This was really a way that NASA, under "Better, Faster, Cheaper"—the idea was that NASA should take on more risk, and should stop only doing really, really big and expensive things. That they could open up an avenue for individuals to propose to do much smaller-scale programs that are more quote-unquote "efficient," understanding that they take on a lot more risk. That's the program that we proposed to. At the time, you didn't have to be a line—there actually wasn't even an Earth science decadal. That's a relatively new thing.

ZIERLER: That's amazing, by the way.

WENNBERG: Because like the astrophysicists and the physicists have done this for a long, long time, but in the Earth sciences, there wasn't such a thing. So, we just out of the blue proposed to map CO₂ from space. This had not been ever suggested before. There's no National Academy report that says, "This is something we should do." To be honest, it was [laughs] a pretty bold idea that by remote sensing, you could actually do this, in the sense that it stretched the requirements to the limit, of signal-to-noise, and of ability to retrieve carbon dioxide or any trace gas from backscattered infrared radiation. This was really quite a claim, that we could do this. So, we started developing this proposal. At the time, it was a two-step proposal. You submit a little teeny thing, and then you submit a much bigger proposal.

ZIERLER: This is kind of like setting up the lab. Don't go all in.

WENNBERG: Yeah. They get a lot of these little guys, and then they sub-select some.

ZIERLER: And the OCO was one of these little guys?

WENNBERG: It got selected for what's called a Phase A study led by JPL scientist David Crisp. You would propose in like a few pages, literally, "Let's map CO₂ from space. Here's the basic idea. And we think we can do it within the budget and time commitment that the proposal allows." Then NASA gets a bunch of these, they sub-selected a few, and then they gave us some money and six or eight months to develop a full proposal, which you then submit. So, between the time we started on this—

ZIERLER: And "we" is who? Who are the key people?

WENNBERG: Oh. So the PI—we had to PI-shop. None of us wanted to be PI.

ZIERLER: How come you didn't want to be PI?

WENNBERG: Oh, it's a terrible job. This is not the job for somebody—I would say this is not a good job for a professor.

ZIERLER: Huh. Because it's like all-encompassing? It takes over your life?

WENNBERG: Yeah. And because you become a manager. A PI of a mission—maybe not of an instrument, but of a mission—is a manager. And we're not good at that, most of us.

ZIERLER: Some are.

WENNBERG: Yeah.

ZIERLER: Some do it very well.

WENNBERG: Yeah.

ZIERLER: That's not you?

WENNBERG: No. And, it's a 100% job.

ZIERLER: Meaning, this is a discussion with the division chair and the provost that you're sort of checking out for a while?

WENNBERG: Maybe that, but more I think it's just mostly internal. You're saying, "Do I really think, given the risks of this mission—?"

ZIERLER: Which are what?

WENNBERG: Well, all the usual risks of space. Add into it the fact that the budgets are really low.

ZIERLER: Faster, better, cheaper.

WENNBERG: Yeah. And then add into it that you're proposing to do something pretty outrageous.

ZIERLER: Why is it outrageous, though?

WENNBERG: Because if you measure to 1%, the sensor is useless.

ZIERLER: So what are you aiming for? What's the percent?

WENNBERG: A tenth of a percent. And even then, that's not even really good enough, unfortunately. Yeah, pretty tough stuff.

ZIERLER: But obviously it happens. You go PI shopping.

WENNBERG: We go PI shopping, and David Crisp puts up his hand. David Crisp was a scientist at JPL and had been involved in aerosol remote sensing.

ZIERLER: Had you worked with him previously?

WENNBERG: No. I didn't really know him. But all of us—the group of five, as I'll call it—we had an idea for how this sensor could work. We talked to Dave Crisp and said, "This is what we're thinking." He says, "Oh, that will never work." He said, "The reason this will never work is because of"—airglow, as it's called, in the upper atmosphere. The way the sensor works is you measure the optical depth—you measure how much CO₂ there is between the Sun, whatever scattered the light, and the spacecraft, so, looking down. Then you also measure oxygen, which although there's no pure rotation-vibration transitions that are allowed—there are electronic transitions in the near infrared, and so you can observe oxygen at these wavelengths, so oxygen—what's called the A-band. Then the B-band. We had proposed, because it was technically a lot easier, to measure oxygen at 1.27 microns. But it turns out—and Dave Crisp knew this—that at that wavelength—O 1D chemistry, so excited state oxygen atoms produce—they react, and you end up producing the upper state of that transition, which then fluoresces. So you chemically produce excitation that leads to fluorescence in the upper atmosphere.

ZIERLER: You're trying to do this.

WENNBERG: No. [laughs] In other words, you're looking down at the earth, and all you want to do is good, honest to goodness Beer's law absorption spectroscopy, but what you have is the source of those photons in the upper atmosphere. So now, it's a true—well, Dave told us it would never work, and so that we should use the oxygen A-band, which unfortunately is further away in wavelength from the CO₂ band, which makes it technically a lot more challenging, but doesn't have this airglow problem. So we said, "Oh, good. You can be the PI." [laughs] "You clearly know what you're doing. You can be the PI." That's how we ended up building this. But, going back to my first story, as we were developing the proposals, one of our group and me—so Geoff Toon, who is an atmospheric spectroscopy expert at JPL, and I decided that for the proposal, we needed a figure that shows that you can do this from the ground, at least that you can measure the total column of CO₂ from the ground with such precision. That led us to first use some archived spectrum from the Kitt Peak Observatory, at the right wavelengths, and show that it works. Then we also got some seed money from NASA to build a dedicated instrument to make this work and show that it works. So by the time the proposal gets selected, Geoff Toon working with one of my first graduate students, Zhonghua Yang, had demonstrated that you could provide a dataset for which to validate and compare the observations from space. That didn't exist before.

ZIERLER: So, you're feeling good at this point.

WENNBERG: Well, we were feeling like—I'm feeling like, oh, I can work on this ground-based network, and there will definitely be science, good science that comes out of it, even if the spacecraft never works. Because it's telling us a lot about the dynamics, by measuring the column for CO₂ is. So, we start building this out, and a lot of people join. It's an exciting time for a lot of other groups.

ZIERLER: Tell me about the instrument.

WENNBERG: The ground based spectrograph is is nothing terribly exciting. It's a one-meter Fourier transform spectrograph, commercial, modified slightly.

ZIERLER: Off the shelf basically, with a few modifications?

WENNBERG: With some money, yeah. Not cheap. [laughs] We built around it all the automation. Another part of it is it has a solar telescope that you mount on top. A one-meter spectrograph is not a teeny thing, so we build these into 20-foot shipping containers. We put steel reinforcement on the roof, so we could mount the telescope. We put it inside of just a commercially available amateur telescope dome, which we buy from these guys in their garage. Then we – I should say one of my very talented graduate students Rebecca Washenfelder together with a JPL scientist Jean-Francois Blavier - assemble around it all the automation so that no one has to be there. We move this thing to Wisconsin, where it becomes the first dedicated site, funded by NASA, to measure the column. We did it in Wisconsin because—well, Wisconsin is flat, so they—public television requires a very big tower. So they have a 450-meter broadcasting tower.

ZIERLER: Whoa! That's tall.

WENNBERG: Yeah, really tall. And NOAA had been measuring CO₂ on a number of locations up this tower for the last decade at this time. This is 2004? So, at the base of this tower, we put our automatic instrument that became the first part of the Total Carbon Column Observing Network. We, as in Geoff Toon!, developed the software for doing all the retrievals here.

ZIERLER: Back here on campus?

WENNBERG: Yes. And I had a couple of really talented people. Debra Wunch, who is now a professor at University of Toronto, and now she's the chair of the Network.

ZIERLER: So this is ongoing.

WENNBERG: Yeah. Coleen Roehl, a member of the staff and who also began her career as an atmospheric chemist, is responsible for running all of the Caltech sites.

ZIERLER: Oh, wow. That's a sign of success.

WENNBERG: Yeah. And actually, all the data is hosted by our Library.

ZIERLER: The Library!

WENNBERG: Yeah.

ZIERLER: Wow.

WENNBERG: Through CaltechData.org.

ZIERLER: Very cool. And it's publicly available? You can go look at it?

WENNBERG: Yeah. You can go and plot data out. And we do not just CO₂ but all the major greenhouse gases. Because we're taking spectra between one and three or four microns we can observe many different gases.

ZIERLER: The concern about 1% and then a tenth of a percent, this is resolved?

WENNBERG: Yes. So now, OCO-2 does probably a tenth or better. Let me go back. I go to New Zealand on sabbatical to help set up another one, down there. The second one is at exactly the same latitude, just in the Southern Hemisphere.

ZIERLER: This is like a six-month thing, or you're there for a little while?

WENNBERG: Yeah, six months.

ZIERLER: Where were you?

WENNBERG: This is in Lauder, New Zealand, which is on the South Island. They already had a research station with a Fourier transform spectrograph. And they had a great—the research station there was very well organized with really good, talented, technical and scientific staff. All they needed to do was convert it to from the mid IR to the near IR to do this. So, worked with them. Got that going.

ZIERLER: Did you have a good time in New Zealand?

WENNBERG: I did, yeah. A lot.

ZIERLER: Gorgeous, right?

WENNBERG: Family loved it.

ZIERLER: Oh, you brought everybody?

WENNBERG: Yeah, we all lived in Dunedin. This was also where I met my friend and longtime collaborator Henrik Kjaergaard, who at the time was a professor of Chemistry at Otago University and is now on the faculty of University of Copenhagen. Henrik hosted Cheryl, my second daughter Emma, and me for my 2nd sabbatical in 2016. We have co-authored numerous papers about photochemistry combining work in my laboratory with computational chemistry from his. Our families became very close during our time in New Zealand.

ZIERLER: Is there a New Zealand NASA equivalent that is the point of collaboration?

WENNBERG: NIWA.

ZIERLER: This is a 50/50 NASA/NIWA—?

WENNBERG: No, 100% NIWA. I was mostly there just to wave the flag and encourage. It's not like I went there and I showed them how to do this. It was as much just motivating them to do so, and helping them. We actually sent them a bit of some hardware that was necessary that we had around here on campus I could scrounge up.

ZIERLER: The motivation is simply to get fuller, global coverage?

WENNBERG: Yep. Start building it out.

ZIERLER: How big is the network now?

WENNBERG: It's about 25 instruments.

ZIERLER: Oh, wow, so it's pretty evenly distributed at this point.

WENNBERG: Eh, there's some missing—there's some big holes, like Africa, India. But you can go on the network and have a look at it.

ZIERLER: Is this an ongoing concern, to build up the network?

WENNBERG: Yeah, I'd say so. Again, I've stepped back from leadership in the last couple years, and now the next generation are carrying it forward with a lot more energy again, so that's good.

Building Out the Ground Based Network

ZIERLER: You mentioned OCO was the forerunner to—I don't know if it's TCCON, or T-CCON. How do you say that?

WENNBERG: Yeah, that's the ground-based network. We built that out as part of getting OCO funded, really. It was, "Let's do this from the ground. Let's show that it works." It's Figure 1 in the proposal kind of thing, that you can do this from the ground. I'm not sure it's really Figure 1, but it's a very prominent figure that shows that this is successful. Then we built out the network, TCCON. The model for the network is completely distributed, so it's funded by local funding sources. We only provide the software. And then as a group, there's a whole like, how do you do this, what are the procedures, what are the standard operating procedures, how do you make this network robust and precise. So, all of that is done by community discussions on a—it was a wiki at the time. I guess today it would be a Slack page; I don't know. But there's an extensive history of developing this network out by involving a community—not of citizen scientists in this case, but of atmospheric radiative retrieval people. So there are a bunch of sites in Europe funded by the Europeans. There's the sites in Australia funded by the Australians. Then there's the U.S. sites that were funded by NASA. Canada has a couple of sites funded by their Canadian Space Agency. Japan has three or four sites, again, that were funded through JAXA and through the National Institute for Environmental Studies there. But everyone works together in a collaborative way. There's actually two sites in China now. So it has really evolved into quite a thing.

ZIERLER: A reflective question on the entire observatory research. As we've talked about before, the balancing act between uncertainty in climate science, being honest with where the uncertainty is, the political pitfalls of that becoming a distraction because the bigger story is certainty; the uncertainty is in the detail. So, twentyish years out, what has this done in terms of tipping the balance toward more certitude?

WENNBERG: That's a really good question. What I would say is that the TCCON—sorry, David—can we take a quick break?

ZIERLER: Yeah.

[Break in Audio]

ZIERLER: We're going to pick up on this idea of balancing the uncertainty and the specifics but the bigger picture, you know the causation is there. The retrospective question is, twenty-something years, TCCON, OCO, how has it contributed to that balance?

WENNBERG: Let me go back a little bit and tell you what happens next, before we get there. Let's put that one aside. What happens next is NASA selects this proposal, the Orbiting Carbon Observatory. About a year later, the Japanese start on their own CO₂ sensor, using slightly different approaches but essentially the same idea.

ZIERLER: This is welcome? This is competition?

WENNBERG: This is welcome, I'd say. Given the technical issues and everything else, they end up getting a ride to space a year before we did. So, the Japanese actually become the first to gather these data, from space. We had had a data-sharing arrangement, that actually I helped negotiate—which is kind of funny, because—anyway, it turns out it was hard for NASA to negotiate something like this, and so they turned to me, and I got to go and spend a fair bit of time wearing an ill-fitted suit, meeting with the Japanese Ministry of the Environment, and others, to negotiate a data-sharing—Caltech was being ridiculous, in terms of the lawyers here, also. The Japanese wanted to put a six-month dark period in, so they would share the data but we couldn't publish for six months. Apparently, this is like a no-no for Caltech. But the Japanese didn't understand what we were objecting to.

ZIERLER: What's the issue with Caltech?

WENNBERG: We don't make agreements that withhold data. Because if your student's thesis involves data, and you're not allowed to publish it, that's really bad.

ZIERLER: This really gets to the institutional ethos of Caltech.

WENNBERG: Yeah. And it's fine. But also, it's also stupid, right? Because six months, no one is writing a paper—

ZIERLER: [laughs] Fair.

WENNBERG: —and getting it published in six months. Also, the Japanese didn't understand why we were objecting. "Why would you object to this? We're giving you this for free." Yadda, yadda, yadda. Caltech is going, "Duh duh duh duh." So, I had one of those like Lost in Translation sort of—where I've got the Caltech lawyer on the phone, and I'm talking to the Minister of the Environment, and I'm trying to explain Caltech's objective. So, I end up writing some language that everyone agrees. I'm not a lawyer at all, of course. Anyway. So we get this data sharing, and OCO launches, and it goes instantly into the drink. So it does not—it's not a successful launch. The fairing fails to come off the payload. The fairing is the cover that goes over the payload, the top of the rocket. In a truly scandalous way, the fairing does not come off the rocket, and so we never make it to orbit. We land in the Southern Ocean, and that's the end of the Orbiting Carbon Observatory. So. The reason why it's a scandal is because NASA then launched a second of these, a different Earth-observing spacecraft, and it has exactly the same problem. And it turns out it was fraud by one of the vendors of the cables that were used to hold the thing together. This vendor didn't even come forward, in between, to say, "I think it's us." These guys who should have gone to prison, really, as far as I was concerned, get away with a teeny little slap on the wrist.

ZIERLER: Wow.

WENNBERG: Anyway. That's an aside. But the good thing was, we had this data-sharing agreement, so this whole team that the project had stood up to do this very complicated retrieval of CO₂ mixing ratio from these spectra got instant access to the Japanese spectra. And so, for the next few years, we were able to essentially generate our own remote sensing product from their spectra.

The Value of the Japanese Partnership

ZIERLER: This is an instant victory.

WENNBERG: Yeah. And keep the project going, really. And learning a lot along the way. We taught the Japanese a bunch of things. They taught us a lot of things.

ZIERLER: Like what?

WENNBERG: Well, the initial retrievals were slow and terrible. It was really bad. And so, all the things you learn when you've—it's always a problem—when you start out thinking, "I'm going to do something," you have some model in your head, some forward model, or in the computer, of what the spectra are going to look like. They're based on all the knowledge you have, of everything in the atmosphere. Then, you do a retrieval of those, so you turn around and say, "Okay, if I had these spectra, what would I get, if I try to turn this around and now do the inverse calculation?" And the answer will always be way, way more optimistic than you'll get in reality. Because your forward model and your inversion model are the same. So now you've got this like—the errors in the inversion and the forward model cancel. And that never happens, because the atmosphere gives you what the atmosphere gives you, and the surface gives you what the surface gives you, in terms of its reflectivity.

So over the first few years, we learned about all the problems associated with complexity of aerosol scattering, of the way the surface behaves in terms of its reflective properties in these wavelengths, about the instrument calibration and how complicated that is, and how it leads into these inconsistencies. And, all this time, we have an enormous amount of the goodwill of the Japanese as well, because we are the ones providing the ground-based data for them to evaluate against. So, the development of the ground-based network, which had been initiated by NASA and then grown by all these other people joining, produced a dataset for which they could then evaluate all of these spectra, both by the NASA group that was working on the retrievals and by the JAXA group, or the NIES group, the Japanese group doing the retrievals. So, we learned a lot. At the same time—so the launch failure was at the time of the last big recession.

ZIERLER: 2008.

WENNBERG: 2008, yeah. You'll remember there was an imperative to spend money at the time. It was, Barack Obama comes in, who is now climate-forward. But at the same time, there is also this "shovel-ready"—do you remember "shovel-ready projects"?

ZIERLER: Sure.

WENNBERG: Yeah. So we put our hands up. We said, "We have one. Let's rebuild this Observatory."

ZIERLER: Was Jean-Lou Chameau, who was very much of that mind about spending and there's opportunity in crisis, was that relevant? Did that help supercharge things at all for you?

WENNBERG: No, no. This was really just a luck of the—that as much as you don't want a recession and all that, but it was really just a coincidence again, that all of a sudden now, NASA had extra resources, and they could put them towards rebuilding and relaunching a new Observatory, which we did. So the Orbiting Carbon Observatory 2 was constructed and launched, but this time, not as one of those cheaper, better, faster things, but as a slower, more expensive, and normal thing.

ZIERLER: Spend and do good science.

WENNBERG: It was built as a NASA-directed mission. And the distinction here is—the OCO, the original one, was what they called a PI-led, so the PI has the budget and manages the whole thing. That's why I was saying you don't want to be a PI of one of these, because you really are the manager. As opposed to when you're the PI of a NASA-directed program, NASA is the manager. They hire you to lead the science, and they hire someone else to lead—

ZIERLER: You're executing their mandate.

WENNBERG: Yeah. But also, they specifically manage the program, and they were the ones who would hire the project manager, who is making sure that all the hardware gets developed. As opposed to the other one, where they just hire the PI, and the PI is now responsible for everything else. So, it's kind of a different beast. So, we rebuilt this, and it was launched successfully and has been getting data since launch, or right after lunch. Now, together, OCO-2 and the JAXA GOSAT mission—that's what it's called—and GOSAT-2—are gathering and continue to return data on CO₂ globally. At the time when it became this NASA-directed mission, the budgets went up a lot. The second version was a lot more expensive than the first. But one of the things they did was they built a spare, and once OCO-2 was launched and was successful, the spare was packaged and deployed onto the Space Station. So today, on the Space Station, OCO-3 is doing what are called targeted measurements of cities around the world, looking at the global emissions from urban areas and other things, again, based on all the work that had been done for OCO and then OCO-2, and shown that this was possible, and that you could learn about these emissions. So that was the structure of this whole thing. Through it all, I have had just some really talented students and postdocs who have really managed to build up this whole ground-based network, do a lot of amazing science, go on to other faculty positions, and so forth. As I mentioned earlier, Coleen Roehl, who joined my group as a postdoc just after I came to Caltech, is now a staff scientist and she oversees most of the Caltech and JPL instruments in TCCON, and keeps the data flowing to the archive.

ZIERLER: Let's get back to the question about reducing uncertainty.

WENNBERG: Yeah. What did we actually learn? We've learned a few really important parts. We've really done a much better job now of measuring the uptake of the carbon by the forests. So now we have a much better understanding of those dynamics, and understanding how the climate is influencing those. That's work I continue to do right now. Just this last week, the first analysis of the net carbon emissions from basically all the world's countries, was released, and it's based on remote sensing, and in situ data, of CO₂. So this data is now informing policy about the trajectories of CO₂ emissions from lots of countries. The hope, of course, is that we'll now see those going down. That we'll have an observational system capable of evaluating whether the emissions in China are what they say they are, and that their trends are what the Chinese say they are. Likewise for the U.S. and Europe and so on.

Maybe we talked about this—there's methane as well. The Japanese spacecraft also measures methane; the OCO-2 doesn't. But this has really led to a—this idea and these approaches for measuring trace greenhouse gases has really blossomed into a large program both in the U.S. and in Europe and in Asia, to measure greenhouse gases from space. In the context of methane, which comes from lots of different sources, these data are now really showing up as impactful in terms of policy relevance. Because if you're an oil and gas emitter in the Permian Basin, say, and you're drilling for oil there, you now have to look up—somebody is watching you—in a way that no one was watching before. These oil and gas companies had to report their emissions—

ZIERLER: So there's a direct line to mitigation.

WENNBERG: There is a direct line.

ZIERLER: That's great.

WENNBERG: What's also cool about it is that that line doesn't even have to exist in reality. It just has to exist in the minds of the people doing the work.

ZIERLER: There's a new regime that's watching now, and that changes things.

WENNBERG: It does change things, yeah. Just this week, there are several oil companies that are in legal trouble now, because there were observations of enormous leaks that had come from some of their facilities that they didn't report.

ZIERLER: Busted.

WENNBERG: Busted. Exactly. Busted.

ZIERLER: In the old days, you could do that, and who's going to know?

WENNBERG: Who would know? Yeah. For CO₂, as you point out, it's much more complicated, because, well, all of us are emitting CO₂. The biosphere is emitting and taking it up. But nevertheless, I think this is a regime shift. This is one where you cannot just simply tell lies and expect that you will not be busted.

ZIERLER: Is there a Perry Mason moment for you, as an expert witness?

WENNBERG: [laughs] No, no.

ZIERLER: "For shame! For shame!"

WENNBERG: Exactly. [laughs] I have a colleague here who I should also talk a little bit about. Christian Frankenberg is a professor here who is incredibly inventive. One of the things that he has been—he has been called or asked to be an expert witness like this, exactly. But Christian—I should back up. Christian was a JPL scientist working on OCO-2 among other things who now is a professor here and I am super happy that I helped recruit him to campus. His most impactful contribution that just continues to give out enormous dividends is the measurement of fluorescence of plants. Did we talk about this last time at all?

ZIERLER: No.

WENNBERG: We'll do the technical weeds thing, because it's kind of fun. If you look at the spectra of reflected sunlight, where the oxygen absorption is in the so-called capital A-band—this is the Fraunhofer A-band—and you do a radiative retrieval, so now you're just trying to fit this spectrum, you'll notice that when you go over forests, there's a zero level offset, namely that there's signal there that appears to shift the whole spectrum up off of zero. That signal is from chlorophyll fluorescence. So, what you're actually looking at are—so, there's the oxygen lines, of course, and they get treated—the fluorescence gets half the optical depth that the atmosphere does, when you're going through it and then reflecting back through it. Because its origin is at the surface now. But more importantly, there's also solar lines, telluric—telluric lines are the oxygen lines, but there's also a number of solar lines that originate in the solar photosphere by absorption by elements and molecules. If you think about those lines, those reflected lines, they should be just as deep in reflection as they are in transmission. If you were to stare at the Sun and look at these lines, and there's no atmospheric absorption, they should look just the same in reflection. But they're the ones that get stood off, from this fluorescence signal, because of course chlorophyll doesn't have these elements of the photosphere. So, you can quantify the fluorescence of plants.

What's super cool about this is that it's directly—or almost linearly related—to the chemical work that plants are doing. Up until Christian invented this, along with a couple of other people—he would be more generous than I am, in attribution—but until this time, we knew what we knew about the global distribution of plants by looking at the global distribution of chlorophyll from color images taken from orbital imagers. You just look where it's green, and where it's not green. What you don't know from any of that is how much work are those chlorophyll molecules doing. For example, an evergreen tree in the winter has chlorophyll, but it's doing no chemical work, because it's down-regulated. And if you look at the fluorescence, there will be no fluorescence in the winter, as well.

ZIERLER: What is the opposite of down-regulating? Up-regulating? Or just regulating?

WENNBERG: Regulating, yes. They basically have a way of turning off—because you don't want to be generating, reducing power, in a leaf, when you can't actually use it.

ZIERLER: This is plant hibernation, basically.

WENNBERG: It's plant hibernation. So, on a hot afternoon, the plants also down-regulate, because they're trying to protect their water sources, and again, the fluorescence signal goes down in the afternoon. We now have this new measure of the productivity of the planet, what is called the primary productivity of terrestrial plants. And it's just hugely impactful. It was made possible by OCO. So, OCO-2 took these high-resolution spectra in this region where you could see this plant fluorescence. By coincidence, it's at the same wavelength as the oxygen absorption.

ZIERLER: Why is that valuable?

WENNBERG: Basically, we had to have oxygen to do CO₂, and as an inventive person will do, they said, "What else can I do with this spectrum?" What Christian showed was that you can really, really accurately measure the fluorescence of plants there. So, super cool. I use his data for my science. It's probably the most important product that came out of that mission.

ZIERLER: Oh, wow.

WENNBERG: More so than the CO₂ itself.

ZIERLER: To go back in the chronology just a few years, the MacArthur Fellowship, did that give you a chunk of funding to be adventurous with? What did you do with that?

WENNBERG: [laughs] This was a magical thing, because it's a gift to you, not to the Institute. It just is a chunk—

ZIERLER: You can use it as a down payment on a house, if you want.

WENNBERG: Correct! I did.

ZIERLER: You did! [laughs]

WENNBERG: Yeah. When Cheryl and I came here, we didn't really have much, as one wouldn't.

ZIERLER: Postdoc coming to Pasadena; something's gotta give.

WENNBERG: Yeah, so we used it to definitely improve our living standard. I gave a bunch away to the—I think I told you I was a musician as a young person, so I gave to the Pasadena schools, to buy instruments. Mostly as a just like pay it forward kind of thing.

ZIERLER: The idea is, the research at this moment in time is not—

WENNBERG: I had all the money I needed. [laughs] It was more—time.

ZIERLER: Including postdoc and graduate funding. These were good times.

WENNBERG: These were good times, yeah. I've never had bad times.

ZIERLER: Thank goodness.

WENNBERG: Knock on wood. I've been fortunate, again, to have come into science at the right time, and also to have just been greatly benefited by my collaborators.

An Inflection Point for Climate Awareness

ZIERLER: The last topic I want to touch on for today goes back to this zeitgeist issue of historical inflection points in climate change awareness, specifically how students, undergraduates, are really an engine, that the Institute really cares about their perspective, cares about their concerns. I'm thinking 2005, Hurricane Katrina, Al Gore and An Inconvenient Truth.

WENNBERG: Right.

ZIERLER: From your vantage point as a mentor, as a teacher, as an advisor, what are you hearing from the students, and is there a way to incorporate those concerns, those dreams, into what you're already doing?

WENNBERG: I think it helps motivate you, just in general, if you're working in climate-proximate work, and there's a group of young people around you. Because of course they are the ones who will suffer and who have most at stake. They do always provide that extra kick, like, "You should do something about this." Personally, though—one of the first classes I taught here—it was begun by Andy Ingersoll, I think. Andy is a planetary scientist but had been a major part of the formation of the Environmental Science and Engineering program, and he taught this class, ESE 101, which was basically Introduction to Atmospheric Science, here. I inherited that class from him, so shortly after I got here, I taught it for many years, until Tapio Schneider was hired. It was an exceedingly interesting, exciting time to teach climate, and also to be able to tell the students, "We've had all of these predictions about what the future looks like, and you're going to see them now. They're emerging from the noise." There's a lot of internal climate variability, right? Just look at, say, the amount of rainfall in Pasadena from year to year. Now you're trying to—

ZIERLER: For example, this year is throwing everything off.

WENNBERG: Yeah. So now, you're looking at trends. And so, trends of course emerge slowly in the context of large internal variability. But what was very clear was that we were living right at that cusp, where the data and the rate of warming—because just look at how much it has warmed since 2000.

ZIERLER: These are real numbers.

WENNBERG: This is stuff you notice. This is no longer the like, "Maybe I can see something, and maybe in a few more years—" You're telling students, "All of these predictions, we will now be able to evaluate them, in the coming five, ten years. And if what we think is going to happen happens, this is not going to be like—it's going to be obvious that we are massively torquing the climate system." So, that was really, really exciting. Then most recently, I started a new class, ESE 1, which is an Introduction to Climate for the freshmen and sophomores. They take these menu classes, as they are called, so there's I think five or six of them that they have to choose one of. Partly because of the way I like to teach, I have limited the enrollment. I started out at 30—

ZIERLER: You want to go seminar style, or as close to it as you can?

WENNBERG: I'd like it to be more personal. I also like the students to be able to do some project-based learning and other things that just make it more difficult as the classes get bigger and bigger. But I also feel—every year I increase my bandwidth a little bit, so I've increased the enrollment. What I've noticed is just the incredible interest in the topic, and it becomes like—I don't know if it's a moral issue to not allow people to learn about this that has led me to be much more empathetic when they send me their sad stories, but we're at 50 this year, just to start in a couple weeks.

ZIERLER: Are the students' facility and excitement about computation, machine learning, AI, is that bubbling up to you?

WENNBERG: All that, yeah, and beyond. One of things I do in this class that the students really like is the last—it's about two-thirds lecture and recitation sections on climate science. So, I teach two hours a week. Then the students all meet in small groups with either me or the teaching fellows and work problems together. Then the last few weeks of class, though, is a chance for them to explore a topic, something that is of interest to them. I often try to get them to think about something that they are actually passionate about, and then think about the climate laid on top of that. So you get everything from fast fashion, for some people, to like you say, AI, and other things. But during that time, rather than lecture, I invite in visitors, as I call them. We do this remotely. These are people working in climate-adjacent—mostly as an idea of getting—first of all, allowing them to become exposed to a much broader—

ZIERLER: Like journalists, policy advisors?

WENNBERG: Correct, yeah. Diplomats. Climate diplomats. Venture capital. People working in small companies who are trying to have impact.

ZIERLER: And from the students' perspective; make money, save the world.

WENNBERG: Right. And they really respond to those, so it's kind of fun. I get the visitor to send in a 30-minute pre-record of how they became who they are today. So, not like the usual, "Here's the science I've done," but, "How did I get interested in this, and what was my path?" Then they also send in some written materials for them to read, so that's often more of the like technical work they're doing. Then the students in these small groups, the same TA groups, each host one of the visitors, so they have to develop the Q&A. So, the visitors come for an hour, online, and the students host—the students who were involved in that session introduce the speaker or introduce the visitor, and then have a whole set of prompts to go with. It's great. I think it has really inspired a number of our students to think more broadly about how they can have impact beyond just the—"I'm really interested in AI. I'm not really interested in climate." And it's like, "You can do both."

ZIERLER: It's Informatics X.

WENNBERG: Exactly, yeah! This is a place we need you. And the world needs you. And it has really been a fun experience to teach this class, and to build off that energy.

ZIERLER: One isolated item before we end, building up to 2008 and the Linde Center. When you were Secretary of Atmospheric Sciences for the AGU—AGU is enormous. It's very influential. It's old. Was that important for you? Was that a service role where this is a big stage where big things can happen?

WENNBERG: [laughs] I actively campaigned against myself. [laughs]

ZIERLER: That's one of the main qualifications, of course. [laughs]

WENNBERG: No, that's not a huge role. That's a role that is more—it's truly a service role, and one which is mechanically—the major deliverable for that position is the scheduling of the enormous annual meeting. If you've ever scheduled a big meeting, what you know is that—

ZIERLER: It's a nightmare.

WENNBERG: Well, and if you did a good job, no one will notice, right?

ZIERLER: Of course! Because it just happens itself.

WENNBERG: Right! And this was a great schedule, and I got to go to the things I wanted to go to, and I didn't have all these conflicts, and blah blah blah. So, it's a terrible job. You basically only hear when you fucked up. And so, yeah, that wasn't a—

ZIERLER: That didn't move the ball forward in terms of the policy or the science?

WENNBERG: No.

ZIERLER: Is there any takeaway, the fact that AGU is now thinking about climate science? What's the—?

WENNBERG: The geosciences, of course, have been the leader in climate. This has been where the conversation and all the work has been. The AGU has become more and more politically active around climate, as a promoter of both the internal interests of the geophysics and geoscience in general, but also helping to drive national and international conversations around mitigation. So, the AGU is important, and has been important, personally. I have come to somewhat disengage, mostly because the big, huge, annual meeting, which is really what AGU is primarily about, I just don't like, anymore.

ZIERLER: Culturally?

WENNBERG: I don't know. You've been to big, big things.

ZIERLER: It's just too big. It's not about AGU? It's just too big.

WENNBERG: It's just like a—I get people-tired fast, and—

ZIERLER: Vermont farm boy.

WENNBERG: And as one will, when you go to just a huge meeting like that, it's exhausting, and I feel like—so I really—I go to Gordon Conferences, which I find—

ZIERLER: Much more boutique.

WENNBERG: —much more personal, and afternoons off to go walk in the woods and talk to people. I just feel like the balance between small discussions and really first-rate science is much more exciting to me than hundreds of 12-minute talks that quickly lead to exhaustion.

Embracing Sustainability Research at Caltech

ZIERLER: I promise, really, last question, now. Just to build the narrative—we'll pick up on this for next time—2008, the origins of the Linde Center—in my mind, I'm thinking there's President Jean-Lou Chameau and this emphasis on high-impact, societally relevant science.

WENNBERG: Yeah.

ZIERLER: There's what you are already doing. And there's Ron and Maxine, who are the best kinds of academic philanthropists, where they just want to be responsive to what Caltech wants at that moment.

WENNBERG: Right. And don't forget Carol Carmichael, too.

ZIERLER: That's what I was going to ask; what are the missing pieces there?

WENNBERG: Yeah, Carol.

ZIERLER: Who is that? Who is Carol?

WENNBERG: That's Jean-Lou's wife.

ZIERLER: Oh! Okay.

WENNBERG: Eminent scientist as well. Very much in this—

ZIERLER: What's her field?

WENNBERG: Environmental science. And so, they were—

ZIERLER: She was Caltech faculty?

WENNBERG: Yes. Well—

ZIERLER: She had an affiliation?

WENNBERG: She did.

ZIERLER: She was present on campus.

WENNBERG: She was.

ZIERLER: Not like Kathy Faber, though, perhaps.

WENNBERG: I think we need to go back and look carefully.

ZIERLER: We'll look. [Editorial note: Carmichael served as senior counselor for external relations and as faculty associate in the EAS Division]

WENNBERG: I think she played an important role in the sense that her proximity to Jean-Lou, and you know, being president, that's a huge job. She also was a lecturer in the Linde Center where she taught a course on environmental policy. I think she really was another person who helped steer that whole conversation. And of course the Lindes, together with Ed Stolper and Jean-Lou, working to explain why they should care about this particular aspect, and how they could assist the Institute as they so generously have over the years.

ZIERLER: From your perspective, the source of their wealth from Envirodyne, that was really not a factor? That did not make this a logical end point for their generosity?

WENNBERG: I should ask the Lindes, but I don't think so. No. When you talk with them, they want to know how to help Caltech.

ZIERLER: That's exactly what I got from Ron.

WENNBERG: They just want to help Caltech.

ZIERLER: Okay, so who moved the ball first, from your perspective?

WENNBERG: From my perspective, it was almost certainly—when I came here, which was the start of this joint E&AS, GPS, program, I was told, "Oh, you're going to get a building."

ZIERLER: That was already in the works.

WENNBERG: And, "You're going to get a place where all of these people who are all spread out over campus can be together." I said, "Oh, that sounds great." And it never happened. For reasons. It's hard to raise the money for buildings and so on. But what did happen was through the advocacy of PMA, they got their new building across the street here. and this derelict building became available. It was just sort of fenced-in, right? I think it was just such a hugely—it was a perfect time. Again, another one of these lucky things, right? That this facility, at least the external part of it, became available at the time when we could ‘call in our chits'. And Ed and Jean-Lou could have access to someone like Ron, and say, "Look at this opportunity. We have an opportunity to do something really special. The costs are high, but it's not like these $100 million that we're spending now on buildings. And it's a beautiful building, with a great cultural heritage for the Institute. And you can have your name on it." I think it just led to a level of excitement. Personally, I met with Ron and Maxine many times, and—

ZIERLER: Was your name floated right at the beginning of those initial conversations between Jean-Lou, Carol, and Maxine and Ron?

WENNBERG: I'm not sure, but I wouldn't be surprised. I was sort of the obviously young enough but not too old person.

ZIERLER: And I assume unlike the AGU, you were not actively campaigning against yourself. This was different.

WENNBERG: No, this was different.

ZIERLER: This was special for you. This was a great opportunity.

WENNBERG: Yeah. And they're lovely people. And this was going to directly benefit my research community, and allow us to start attracting new faculty. So, we were able to build the climate program here inside these walls. This is not an uncompetitive field, right?

ZIERLER: Right! And there's always the Caltech, "We're so small, we're so fundamental, where do we contribute?"

WENNBERG: Right. And in the context of climate change, climate science—we started this conversation around, "What was Caltech in atmospheric chemistry and why did you come?" In terms of Caltech and climate, there was little to nothing here.

ZIERLER: Of course. Right.

WENNBERG: So, if you're trying to attract someone, say, Simona Bordoni, or Tapio Schneider, Andy Thompson, Jess Adkins, all these people who we attracted to Caltech, you had to have something special. And one of the things we had special was, it's Caltech. This is a great place to work. The other thing we had was the Linde Center, and just the physical environment of this building.

ZIERLER: It's going to be a critical mass around this one issue.

WENNBERG: That's right. It's not like we're hiring one person—"You're our climate guy." Or "our climate gal".

ZIERLER: I think that's a great place to pick up for next time—2008.

[End of Recording]

ZIERLER: This is David Zierler, Director of the Caltech Heritage Project. It is Friday, May 12th, 2023. I am delighted to be back with Professor Paul Wennberg. Paul, once again, thanks for having me. Good to see you again.

WENNBERG: Nice to see you after a little bit of a pause.

ZIERLER: That's right, that's right. Paul, we're picking up in 2008 where we left off last time. We were talking about one of the things that was attractive was just building the Linde Institute as a critical mass to attract all of the great people who are here now that wouldn't have been possible otherwise. Before we get to that, I wonder if you could set the stage for your own science at the time. What was happening in carbon cycle science, what was happening in tropospheric chemistry, that made the Linde Institute, or the concept of having an environmentally focused science institute at Caltech, how was that personally exciting to you for where your research was at the time?

WENNBERG: First of all it's the Linde Center.

ZIERLER: Linde Center.

WENNBERG: Gotta keep these separate.

ZIERLER: Right, there's so many Linde-supported organizations, yes.

WENNBERG: There's a lot—Ron and Maxine have been incredibly generous. But they've added confusion [laughs] because of all the Linde things.

ZIERLER: The best kind of confusion.

WENNBERG: Exactly, the best kind of confusion. In 2008, two things are happening personally with my science that make helping to create the Center of great interest to me, one of which was we had been working on building new methods to measure CO₂ from space and from the ground. I saw the Center as a place where JPL and Caltech could collaborate on environmental science. JPL has a history in planetary science, but in the beginning of the 2000s, really starting around then, JPL—actually, it goes back even a little bit further, into JPL had contributed a couple of instruments to the large NASA buses. JPL becomes much more invested in and active in Earth science, and in climate science. So, I really saw the Center as a place where we could have real collaboration in the science and technologies for doing environmental science. As we recruited new faculty, they also saw that as quite an attractive thing.

ZIERLER: With JPL as that asset?

WENNBERG: Yeah. If you think about it, if you wanted to attract an oceanographer to Caltech—we have two now, two excellent oceanographers, in the Center, Joern Callies and Andrew Thompson. Andrew has taken over for me as the director. This isn't the first place you would think to be an oceanographer, because there's like no ocean here, right? We don't have ships. So all the classic—

ZIERLER: This is why we missed plate tectonics.

WENNBERG: It is! Yeah! And so, one of the reasons why oceanographers go to institutes of oceanography is because they have ships. But more and more, the tool of oceanography became space-based remote sensing. And so JPL, through a number of different programs and collaborations, built an oceanography group, and built several amazingly important ways to view the ocean globally. And so now we can recruit oceanographers here to Caltech, because this is a center now of oceanography. Pasadena has become one. So that's great. And you really see how this collaboration with the Lab serves as a mutual attractor for good things, for good science, for good people.

The second thing that happened—when we thought about the Center, one of the main ideas was to bring together faculty working in environmental science but who were spread out across the campus. Now, you think, well, Caltech is a small place, and do you really need to be collocated? These are sort of the classic questions. There's nothing like talking to colleagues over coffee, every day. Even more importantly, for your students to interact. Because it may be that the faculty have a faculty meeting and they go and they say hi to each other and they talk about stuff, but for the students to be working side by side is I think really a force multiplier, as it were. And so, something happened that I hadn't appreciated until it actually already happened, which was that John Seinfeld, who had been running a photochemical smog chamber on the roof of Keck—they were doing lots of experiments where you would essentially oxidize, in a large Teflon bag, individual components of organic compounds that get emitted into the air, and look at what happens to the air in the bag, in particular how much particulate forms, how much ozone gets created, these kinds of questions.

This incredibly talented staff member of mine—he was a student and then he became a staff member, John Crounse—he has been the leader of our in-situ instrumentation—was talking with Jesse Kroll, who was a postdoc with John at the time. John said, "I think our technique can measure multifunctional hydroperoxides. This is something brand new—organic hydroperoxides. No one has been able to do this before. Why don't I wheel our machines over there to the chamber and let's see what we see?" So, we did a couple of oxidation experiments that were just incredibly revealing about just the power of combining John Seinfeld's approach with these new machines that we had built and invented, especially John Crounse. Again, it was like, oh my god, John Seinfeld is going to have his lab in this building, in the Center, and this is just going to create lots of these opportunities for collaboration, where we put together the talents and capabilities of both groups to do and learn things. And so, John and then several students and postdocs, especially Fabien Paulot, who was a graduate student in my group—when he first arrived, he was going to work with Mike Hoffmann on water chemistry, but our program for graduate students is such that they work with two faculty during their first year. Usually it's like the main project, what they think they're going to do, and then some important side project.

ZIERLER: For exposure.

WENNBERG: Yeah, so a rotation. But they actually get grilled on both projects at the end of their first year as that's their qualifying exam, and they're equally important. So, they do work hard on both. This data that we had taken over at John Seinfeld's chamber had a number of mysteries in it, so I said, "Fabien, why don't you figure out what's going on? We're kind of busy with a lot of other things. Maybe you can figure out what these signals all are, and what they tell us about this chemistry." There began Fabien's thesis. He ends up basically rewriting how the major emission of trees to the world's atmosphere, what happens to it. There's a compound called isoprene. It's a very, very reactive alkene, dialkene, that is synthesized in the trees, and they emit it to the atmosphere in large, huge quantities—more than methane gets emitted to the atmosphere. It has a very, very short lifetime, because it's very reactive. And so, it influences air quality in a city like Los Angeles, because all these trees out here are emitting lots of isoprene. It affects how much haze there is in the southeast United States. If you think of the Smoky Mountains, isoprene is one of the major contributors to why the Smoky Mountains are smoky. And Fabien and John Crounse pretty much figured it out. Figured out what happens to that molecule when it's emitted in the air, and how does it end up producing aerosol.

What they discovered was that the chemistry, away from intense nitrogen oxide, NOx sources, leads to the formation of epoxides, organic epoxides. They had not been known to be created in the atmosphere. And just like with the epoxy glue—so you have the white, stinky stuff; that's the organic peroxide. Then you have the yellow that that you mix together. The yellow thing is hydrochloric acid, and so when you mix it together, what happens is the acid causing the ring opening of these epoxides and their polymerization. That's how you go from the goo to a hard bond. This is what happens in the atmosphere. You make these epoxides, and then they come into contact with an acid on a particle, and you get polymerization of the isoprene molecule forming a lot of organic aerosol. That was work that Fabien, together with John Crounse and others, led, and it's probably the most important single result that came out of my lab, in terms of photochemistry, but maybe even more generally. It really shifted how people thought about—

ZIERLER: It's a foundational discovery about something so basic it's amazing it wasn't discovered long ago .

WENNBERG: It really was. Following this study, many students worked on isoprene chemistry in my and in John Seinfeld's lab. We had fun working together over two years to put together a comprehensive chemical review of isoprene chemistry – including the discovery of epoxidation and autoxidation – that I completed during my 2nd sabbatical in Denmark in collaboration with a very talented Caltech graduate student, Kelvin Bates.

WENNBERG: Again, shifting back now to the Center—

ZIERLER: The Center, as you explained, sort of made it happen.

WENNBERG: It really made this happen, yeah.

ZIERLER: The amazing thing there is just how small Caltech is, and yet there still needed to be a Center to bring these things together.

WENNBERG: That's right. So, over the years, I've just been incredibly enriched by all of the close contacts that we've had here in the Linde—well, I call it Linde Laboratory.

ZIERLER: I wonder if you can talk about, in creating the Center and formulating with your colleagues, how you envisioned the balance between applied science and fundamental science, given the fact that there are so many obvious and super important translational application of the basic science. How did you conceive of those things in the beginning?

Fundamental Research for the Most Important Applications

WENNBERG: Say it again? Tell me again what you're really trying to get at?

ZIERLER: There's the fundamental science for the sake of curiosity-driven science, and then there's the applied science because we've got a real problem on our hands and we need to devise solutions to get there. Climate change, global warming. How did you conceive of—is there a balance to try to strike from the beginning that you thought about?

WENNBERG: I'd say that I'm a very curious natural scientist, but I want to work on applied problems. [laughs] In the end, I think I'm motivated by the fact that it should be useful, the knowledge that is created. I always feel like the first question you ask is—when you're thinking about working on, say, a new molecule or a new system or a new idea, the first question I have is, "Is it important?" And when I say, "Is it important?" I say, "Is it important to someone else?" This idea of just doing purely curiosity-driven stuff, it works for a lot of people, but it's not what gets me up. What's fun, though, is that through the people I've worked with and the tools we've invented, we get to do both! We really get to work on the fundamentals of how molecules react, but we're always looking for, can it tell us something about the way the Earth works? Because in the end, that's what I feel I want to be able to do. I want to be able to explain environmental science.

ZIERLER: What about global warming specifically, as an end result? The general understanding of how the world works; how much of it was geared towards the specific and overriding challenge of figuring out global warming, figuring out ways to mitigate it?

WENNBERG: We would never have even considered the idea of investing all that time in trying to measure greenhouse gases if they weren't involved in what is probably the biggest experiments humans are doing on the environment over the last 150 years. And so, we always look to the idea that we want to both explain where the carbon is coming from, where it's going, and understand then how does that map onto a future. We're working right now on the growth of the Northern Forests, which I think I mentioned earlier. What happens as those forests warm and as they have more CO₂? This is something important because those forests are sucking up a lot of the carbon we emit. You wonder, is that going to change going forward? How do those ecosystems respond to both the extra heat and the extra CO₂? And can we then say something about the fate of those forests and the fate of the carbon? Again, explaining the past, the changes, is part of what we're trying to do, but it's all thinking about, what does the future look like? I want to be able to motivate what I do based on environmental science. Describing the past of the Earth and then what it tells us about the future.

ZIERLER: Tell me just administratively, institutionally, how you built up that partnership with JPL.

WENNBERG: I guess what I would say is, surprisingly, Caltech and the Lab don't collaborate very much, as institutions, even though everyone at the Lab is a Caltech employee. I think it just simply reflects the culture of Caltech, that you don't have a dean telling you, "Oh, you need to do x, y, and z." And so the way collaboration works with the Lab is that you find people there who you want to work with. Or, you have an idea that you recognize the facilities at JPL can help implement, and that you can inspire a mission, say. That's the way I think most collaboration with the Lab works, and that was certainly the way it works here, is that I had, through my work before I came to Caltech, a number of colleagues in atmospheric composition up there, who were scientists. We naturally worked together, and we continue to work together, and we continue to say, "What could we do now? What could we do next?" I think that's sort of the standard approach. And by having the Center here, where we now can bring down JPL folks to help teach classes—they teach several different classes. They can inspire our students to go work at the Lab. A number of our postdocs end up working up there, our students end up working up there. So, we build the collaborations brick by brick, but actually mostly people. In the most recent example, Josh Laughner who took over from Debra Wunch working on the TCCON retrieval algorithm and code has just moved up to the lab.

ZIERLER: I know it was a hallmark of Jean-Lou Chameau's presidency to think about Caltech's impact at an institutional level.

WENNBERG: Yeah.

ZIERLER: Was the Linde Center sort of a happy coincidence, or was there a real connectivity between this vision that he had Institute-wide, and obviously the impact that the Linde Center could have?

WENNBERG: I think it was mostly a feeling like climate and global environmental science questions are at the center—because, Jean-Lou was an environmental scientist and his wife Carol was, as well—are really at the center of the challenges of our times. And Caltech was punching under its weight, in part because of the same thing that makes us so great, which is this lack of coordinated—anything—

ZIERLER: [laughs]

WENNBERG: —right? So you end up with a lot of the atmospheric composition people are in Chemical Engineering. Why? Because of just happenstance. There's no planning involved. And you end up with others in GPS, others in Engineering. And the sum is the sum. I think that was part of the vision for the Center, was to make the sum much bigger than the parts. I think it does accomplish that, again not by telling people to collaborate together, but rather by giving them joint agency to collaborate much more, simply because you're close by. And we got new facilities, got a beautiful building. It makes the place—again, the ability to hire new excellent faculty, because they come here and they go, "This is a really cool place. You guys are actually doing something that Caltech should be able to do, which is work at the interstitial boundaries of fields." Lots of people talk about interdisciplinary science, but Caltech is good at it, because we don't stovepipe people. The missing glue, though, in this case, was the ability to have much more conversation. By putting people together, by creating these new spaces and new facilities, that's what I think the Linde Center did.

ZIERLER: Two buzzwords that are around all over the place today—to historicize them—sustainability and interdisciplinarity. Let's start with sustainability. 2008, around that time, were people talking about sustainability, not just as an ideal but as a scientific framework, the way that that word is used today?

WENNBERG: I don't remember the conversations being quite that way. Certainly on campus, there was interest in waste, right? Certainly the students were interested in, "Can we reduce our waste?" Green chemistry. "Can we use less solvents?" That part of sustainability I think was active. But the idea of sustainability writ large, as humans and the Earth working in a closed system, to be not utilizing stored resources—which I think is what I might define. We're not going to dig up old carbon and burn it. We're going to live with the energy we get from the Sun and from nuclear. I think that's sort of a new framing. I don't remember it being part of the conversation back then.

ZIERLER: Were people excited about the notion of interdisciplinary studies? Again that's a buzzword now. It's presumed. People are coming from different disciplines, so by definition. But was the concept of interdisciplinarity sort of a rocket booster for the Center?

WENNBERG: I would say absolutely, yeah. I also think this is one of the magics of Caltech, is that—I don't remember—Dave Baltimore was like, "Let's make"—he said—he threw it out there as an idea—"Let's make Caltech a lot bigger." Whether or not we could have even found a place to put everybody, but this was his—"Shouldn't Caltech be a lot bigger?" And the answer that was resounding at the time was, no, that what makes—

ZIERLER: We're not MIT.

WENNBERG: Yeah. What makes—and we can't aspire to that, because we're not going to grow ten times. So, what makes Caltech different is that you end up eating lunch with a mathematician, or you end up talking to a whole group of people that may be the key to solving that problem that you have; you just didn't know it. So that really is I think the key strength of Caltech, is the smallness does give us an advantage in that. It does have its costs, and I think you have to balance it against the fact that our Earth Science Division actually isn't that small. Our Chemistry Department actually isn't that small. Our Engineering effort really is that small, right. if you think about Stanford or MIT, the earth science programs here and the chemistry programs here are not that different, but the engineering has a really different flavor here.

ZIERLER: Because of all of the interest from giving organizations—federal, private—all of the interest in climate—was that useful to you, to ride that wave, from the beginning, just from a funding perspective?

WENNBERG: Oh, yeah. I think in general, climate science and the kinds of work we do has been one of the bright spots in federal funding landscape over the last decade. Even through some of the droughts, when federal policies nominally shifted, I think in general the funding environment has been pretty secure for the kinds of work we do here. Not this feast and famine like the biologists have experienced, where they got that big boost, and then—yeah. I think ours has been on a more sustainable pathway.

ZIERLER: [laughs] There you go! You mentioned this conversation and last all the great success in recruitment from having the Linde Center here. Superimposed on this, though, 2008—this is when the financial crisis hits. Were you insulated from that, given the Lindes' generosity, or you felt that as well?

WENNBERG: There's two parts to that story. There's one part which is unfortunate, which is that the Lindes donated the funds to build the Center to Caltech, and Caltech, as it is, sold off whatever it was they gave them—I don't know if it was cash or whatever—and put it into the endowment. The theory had been that the Linde gift would pay bond to support the development of the building, the infrastructure, and then also programming. What happened, though, was the value of the donation went down a lot, and so, in the end, the vast majority of Ron and Maxine's gift for the foreseeable future is paying the bond on the construction project. I think it's a 30-year bond. Typical. [laughs] We're now 15 years through. So, in 15 years, the Center will be wealthy—

ZIERLER: [laughs]

WENNBERG: —in terms of programming money. So that was unfortunate. For the Center, 2008 did have that unfortunate thing. On the plus side, though, personally, the Orbiting Carbon Observatory, the first one, had gone into the drink. It had failed during launch. And because of the recession, the federal government said, "We need shovel-ready." [laughs] "We have one. Let's rebuild this satellite program." And we did, and it launched successfully. So that was completely dependent on the recession.

ZIERLER: This would have been 2009, 2010?

WENNBERG: 2009. Yeah. We basically were able to recover from a tragedy—the loss of the spacecraft—as a direct result of the federal government looking to spend money.

A Sequel Opportunity After Mission Failure

ZIERLER: Was it a total redo? Were there any tweaks that you did?

WENNBERG: It was a complete rebuild, so there was nothing, other than some new detectors. It was exactly the same, with all of its flaws that we didn't know of [laughs]—

ZIERLER: [laughs]

WENNBERG: —because it had never gotten to orbit. The original mission had been done under a program that were called PI-led missions. Cost-capped PI-led missions. So, I wouldn't say it was a shoestring, but it was very, very resource limited, as to what we could do. As a result—actually, down here on campus, we did a lot of things—we had done a lot of things for the mission that you would normally associate with operations from the Center, from JPL. But after the failure, NASA decided that this was actually a very important mission [laughs], so it's not going to be PI-led; we're going to do what's called a directed mission. So, the rebuild ended up costing many times more than the original. Which you would think, "That's ridiculous. You already have all the plans and everything." But they layered on lots and lots of people, lots and lots of management, and it became a NASA mission rather than a PI-led mission.

ZIERLER: Were the science objectives broadened?

WENNBERG: Nope, it was literally a redo, but it was done under a completely different management structure. And I think there might have been some thinking that the launch failure may have had something to do with the lack of resources, perhaps oversight. I don't know exactly.

ZIERLER: What was the effect of NASA's interest and all of the bureaucracy it added to it?

WENNBERG: The resources were way better. You weren't always like trying to dig in the couch for nickels to do stuff. So it made the development a lot less stressful on the team.

ZIERLER: For you, absent from NASA, what were the objectives?

WENNBERG: For the mission?

ZIERLER: Yes.

WENNBERG: For me, it was twofold. I'm an analytical chemist, and I want to know, can we actually do this? That was certainly part of it, was just to be able to demonstrate that you could actually learn about the carbon cycle from measuring CO₂ from space. Because remember, there's only a 1% difference in CO₂ across the globe, so you really needed to nail the precision in order to be able to see the gradients that then tell you about the exchange.

ZIERLER: That means that the CO₂ over Mumbai is going to be the same CO₂ over Siberia, at the global scale?

WENNBERG: Within 1%.

ZIERLER: That's amazing.

WENNBERG: Yeah. Like even over Los Angeles, in the middle of the afternoon, the total amount of CO₂ over Los Angeles is closing in on 1% more than it is off the coast.

ZIERLER: The big story here is that it's just all connected. It's just dispersing constantly.

WENNBERG: Yeah, it's got a long lifetime, and it doesn't rain out or anything. The things that make it a really "good" greenhouse gas—that it has a long lifetime, strong absorption—are the things that make it really difficult to measure, in order to see the differences. Because it's the differences between CO₂ here and there that tell you about the exchange, which is what we're trying to fundamentally get at. How much CO₂ is being put in the air? How much is being taken out? You need to do this to a fraction of a percent.

ZIERLER: At some level, the mission was proof of concept?

WENNBERG: It was.

ZIERLER: You wanted to know what the findings were, but you wanted to be able to demonstrate that you could just do it?

WENNBERG: Yes. And now it's being copied by lots and lots of missions, because we showed that you could do it. So now there's like this—the Europeans, the Japanese, the Chinese—everybody's in the game, to be able to quantify emissions of CO₂ from cities, or to look at how the forests are responding to climate. So, it was a real path—"Can you do this?" For me, that was in and of itself a very interesting thing to work on.

ZIERLER: I wonder, for part two, if you could narrate the process of figuring out proof of concept—"Can we prove it?" What was that like? How do you know if you get there?

WENNBERG: Because we built this remote sensing ground-based network out of Caltech that we could demonstrate was accurate, by direct comparison with the state of the art in situ observations. So we had ground truth. And we had ground truth because we built it here. And we built it here as part of the mission because we knew that the central first-order question is, "Does this thing work?" Normally, if you know about missions, they're often like, "We're going to build the sensor. We're going to calibrate the sensor. We're going to put it in orbit. We're going to get all this data." Duh-duh-duh-duh-duh-duh. "Oh, and we'll have some sort of validation program. Because we have to." But the difference with OCO was that it was completely integrated from the very beginning. We knew that the only way to provide any confidence that this thing is right was to build our own validation program, robustly funded, built out across the world, where we could know whether the sensor was working at the level that it has to work in order to tell us about these fluxes.

ZIERLER: All of this international involvement, all of these other countries, are they present from the beginning, or they see what you're doing and they build their own, or how does that work?

WENNBERG: We built on top of another international network that had been set up over the years for studying stratospheric science. The stratospheric science community historically has had a strong international component, so we could see that this group of people had the right tools and the right approaches to work on this problem. What happened was we showed—we could take the machines that they were using, modify them slightly, and we could get it to work for our problem. And once you show that, they're like, "Oh, we want to do that too." So, very rapidly, you built out this really big international collaboration through the network. The annual meeting is in a couple weeks, and there will be 150 people there. It's in Europe. All the space agencies from around the world will be there, because they're completely invested now in this. So, it has been a real success.

From Investigation to Mitigation

ZIERLER: You made such an important point in our last discussion about the carbon emitters. Now they know they're being watched.

WENNBERG: They're being watched.

ZIERLER: That's a great example of the basic science and its value to building a climate regime and all of this. As a result of the proof of concept—you proved it—how does that then translate to—we understand how these things are working; how does that get us toward mitigation strategies?

WENNBERG: Right. To be honest, there is a mitigation strategy that the Earth has, which is that of the CO₂ we put into the air by burning fossil fuels, about half of it gets taken up by the ocean and the land. So, we are studying that mitigation, the natural—it's not an engineered mitigation, but nonetheless it is a critical part of the service that the Earth does for us, to suck up all of that carbon. This has been a focus of a lot of the work in my group, and in collaborations with others, to try to understand, again, the forests, how they're changing. Where is all that carbon going? So, there's that aspect of it. There's a second aspect, which is just really coming into the fore now, which is that we have a long enough record and we're developing tools that allow you to quantify the amount of CO₂ being emitted from smallish to largish regions. So, Eastern China now, from these technologies, I think you can quantify a change in emissions to probably 10% or smaller. So now if China says, "We are peaked out on our emissions, and over the next decade, we're going to drop by 25%," I think we now have the tools to evaluate such a claim. You couldn't do this for Vermont, because their emissions are already so teeny. So if they said, "We're going to reduce them by 25%," you just wouldn't see it. But all the big major emitters are now—there's now eyes in the sky and an approach that allows you to say whether or not they're meeting their goals that they've claimed.

ZIERLER: The UN must be all over this technology?

WENNBERG: Yeah. Exactly. Through the UNEP—I work with the UNEP on methane, because it turns out that that is an easier problem, in some ways, because the emissions are—

ZIERLER: Unwanted.

WENNBERG: Yeah, and they're also often unknown, and so you're actually discovering bad actors. Recently there was a great story about Kazakhstan. People were annoyed because they were flaring off all their methane. They're generating oil, and they were flaring it off, and people were like, "Hey." Because you can see flares, right? From space. You can see the bright flares of all that gas being burned off. So what they did was turned off the flares. [laughs] And, "Oh, it's beautiful now. There's no more flares." But now you have methane sensors in space, and you go—the emissions from Kazakhstan from their oils and gases produces a carbon climate forcing that is bigger than all of the emissions from the U.K., just to give you a sense of just how intense those emissions are. A decade ago, you would never have known. You couldn't see it. Methane is invisible. But now it isn't. Now it's quite visible.

ZIERLER: Your initial directorship was 2008 to 2011?

WENNBERG: Right.

ZIERLER: Then you took a bit of a break on the basis that you'd be coming back? Or that was a surprise?

WENNBERG: After getting everything started, to be honest, I was a little tired.

ZIERLER: It was a big administrative lift.

WENNBERG: Yeah, it was, but it was also just that I really wanted to work on other things. Also, we had hired and then promoted a fantastic scientist, Tapio Schneider, who, among many things, is a great visionary, and also a great colleague, so he was willing to stand up and take over. He became the director. Then, unfortunately [laughs]—his wife is Chiara Daraio. She is a materials scientist.

ZIERLER: I heard her wonderful Watson Lecture just the other night.

WENNBERG: Yeah, I was there too. It was really good. - Chiara got recruited by the Swiss. It was definitely they were recruiting her, and Tapio came for the ride. So he left Caltech for two years while they went and tried out something else. Fortunately, he decided he liked it better here, and so, they're back.

ZIERLER: This is a not-uncommon phenomenon. You hear these stories.

WENNBERG: Yeah, people go, and come back. It's actually I think almost unique to Caltech, that go-away-and-come-back thing. Because I think few institutions are as secure in their own self that they would allow people to go away and come back, because then you would think everyone would. Why wouldn't I do that? The answer is, because it's really great here. [laughs] So, yeah, we've had many people who have tried out something and then come back.

ZIERLER: On the administrative side, before we get to the science that was pulling you away from the Linde Center, what had you put in place? What was the sense of—not fully accomplished, but that you could hand off the reins to somebody, where it had its own momentum?

WENNBERG: There's the momentum part, and then there's also the new energy. We had invested in a postdoc, so we had a Prize Postdoc funded by Foster and Coco Stanback. A number of programming things we were able to get started. A number of new approaches that people could—we had funds—small amounts, but nevertheless, discovery funds, as we called them—where you could apply and quickly get funding to try out an idea. All of that was up and running. The students through the Environmental Science and Engineering program, we had really created something I think that was much more coherent for them, a new academic program. That included the core classes that exist today that get everyone up to speed on the same boundary. We had done all of that. Then, it was ready for the next level of investment. Unfortunately, though, Tapio was only able to do it for a year and a half or so before he left, or two years.

Atmospheric Investigations Beyond Earth

ZIERLER: And the science? What was so exciting to you circa 2010, 2011? You wanted to spend more of your time away from administrative stuff.

WENNBERG: OCO-2 was launching, and I really wanted to spend a lot of time working on that mission. Then something else happened, which was a diversion, which was that over the years I had poked at the idea of doing some planetary work. There came an opportunity which I think I basically helped create, along with Mark Allen, who was a scientist at JPL, to do atmospheric chemistry at Mars. We had proposed over the years that you could do an experiment that had first been done really by JPL, from the space shuttle, whereby you take very fast, high-resolution, high spectral resolution spectra of the Sun, as it rises and sets through the atmosphere. They did this from the space station, and this is data that is still being analyzed today. Then there was a follow-on free-flying spacecraft that the Canadians built called ACE-FTS. We had decided that this was just right for Mars, because the technologies had moved ahead enough. The data processing. You can't bring interferograms back from Mars because the volume is too big. But the idea that you could acquire and process them into spectra and then run retrievals at Mars [laughs] effectively. And also the data volume could be reduced a lot, so you could bring it all back.

Then there came a mission called ExoMars. ExoMars was a joint ESA and NASA program. It involved two launches. The first was an orbiter, to do communications, but also to do atmospheric science. The idea was to have several instruments on there and then to follow that with a big lander, like the Mars lander from JPL. What came to pass was that because of the way Europe structures the finances around missions, these two launches were basically glued together. The whole thing had to stay together, because of the way they fund their missions. It's a long story but it's basically, if the French government puts in $100 million, they get $100 million of business. And you couldn't do it—the independent launches, you couldn't make it work, pencil out. And so, by putting the two together, you could have the Germans do a lot of the first one, and the French do a lot of the second one, and on average it works out. But the OMB came back and said that the lander was going to be much more expensive than they thought, and NASA didn't have the funds, and so they want to just do the first—just the orbiter. Europe said, "No," and so NASA pulled out.

That's all by way of saying that I wasted $10 million building an instrument for—I was the PI of an instrument on the orbiter - leadership I shared with my Canadian colleague Victoria Hipkin, which we called MATMOS. So, I had sort of not quite shut my lab down, but sort of. I was definitely running it at a much more smaller level in order to lead this instrument development project. And if you know anything about planetary science, with Mars, there's a launch window every two years, and so it's not like you can just delay. This is a real train. Especially because you're on a large bus with lots of other instruments, there was very little room for error. So, it required a lot of my attention, and not particularly fun kind of stuff. A lot of management of people. The fun parts of making sure the instrument was going to work, I still enjoyed, but a lot of it was also just dealing with budgets and dealing with people.

We got fairly far along in the development of this instrument—we bought a lot of hardware; we were putting it all together—when it got cancelled. It was a little bit of a gut punch. And, it was a collaboration with the Canadian Space Agency, so it wasn't just NASA. So this instrument had to go through a fair bit of—let's just say that it required a lot of good collegiality and organizing people to work together across the US and Canada.

ZIERLER: I'm sure you've asked yourself—had it gone through, what would have been achieved?

WENNBERG: It would have been amazing. Yeah. The really neat thing about this technique is you get a full mid IR transmission spectrum of the atmosphere.

ZIERLER: Infrared—IR.

WENNBERG: Yeah. And because the Sun is such a good light source—it's bright, it's collimated—you get incredibly high signal-to-noise, at very high spectral resolution, in a very small instrument. We calculated that you could quantify things like methane down to a part per trillion, of atmospheric composition.

ZIERLER: Whoa.

WENNBERG: Yeah. You could do the isotopes of methane if there were enough. The coolest thing, though, was you don't need to know what you're looking for. You're not building an instrument to look for anything in particular. You're building an instrument that will—

ZIERLER: Allow you to go fishing, essentially.

WENNBERG: Yeah. One of my colleagues at the time said—what was that thing on Star Trek where they had a sensor, and they'd point it, and—

ZIERLER: Oh, yeah.

WENNBERG: —and it would—without knowing what it was, it would tell you what the atmospheric composition was? That was this. It's the ultimate in knowing everything about the composition of an atmosphere. It had been proven on Earth, and it would have worked fantastic at Mars. But I felt [laughs] sufficiently burned by the whole thing that I—that was the end of my planetary involvement.

ZIERLER: Has the technology seen an afterlife in other applications?

WENNBERG: The center of the instrument was a—well, it's a pendulum for a transform instrument, and the company that has developed this and continued to develop it was a Canadian company called Bomem. They got bought out by ABB. But those sensors, that basic interferometer, is now flying in lots and lots of different spacecraft, for doing—it's part of our weather satellite now. So it has found a huge market as far as space-based interferometers go. [laughs] Which is small, but—you know.

ZIERLER: In terms of Mars science, there remains a gaping hole in—

WENNBERG: What the Europeans did was said, "Okay, screw you, NASA." And so they actually got a spectrometer from the Russians. Doesn't work very well, and it doesn't have this simultaneous measurement of the whole spectrum. It's much more like, "We're going to look for this molecule." So, yes. [laughs]

ZIERLER: The obvious question—what are the big question marks about Mars that we should know at this point, if you had to guess?

WENNBERG: We had always said what we wanted to look for was—the Mars atmosphere is oxidizing.

ZIERLER: That's why it's red?

WENNBERG: The calculations suggest that most—like methane would have a two-or-three-hundred-year lifetime, and so the argument was that if we could measure organic trace gases, we could look for their evolution from—they have to be coming from the surface. So it would tell you about subsurface organic chemistry. Maybe life.

ZIERLER: Ah! That's the big question.

WENNBERG: That was sort of the idea. Is there a small residual biosphere on Mars that's hidden? And could you observe its impact on the trace gas composition? Everyone says—like Sagan, right—you remember Carl?—he said, "If you're on some planet far, far, far away, and you look at Earth, you would know that there's life on Earth, because you simultaneously have an oxidizing atmosphere and you have a lot of methane." And that is just impossible without a large source of reduced gases, basically. So, our theory was, well, maybe—we know there's not any huge biosphere on Mars [laughs] but could we detect a residual one by having this magic sensor?

ZIERLER: These seem like more profound questions than even perhaps sample return.

WENNBERG: [laughs] I don't want to be comparative, but—yeah. To us, it seemed really important. That maybe you'd make major discoveries about—

ZIERLER: I just mean the capacity to go deeper than these small drill core samples.

WENNBERG: True. The other thing we wanted to do is that there has been a lot of questions about vulcanism on Mars as well. So the same idea—could you detect residual magmatic processes as well? That was the basic theory of this thing, which was that there might be imprints in the composition of the atmosphere that tell you about subsurface processes that you cannot see and don't have much hope of seeing.

ZIERLER: Including the existence of water.

WENNBERG: Yes.

ZIERLER: Because that's where all the excitement is. If there is current life on Mars, that's where it would be.

WENNBERG: Right. So, that was the idea.

ZIERLER: Oh, man.

WENNBERG: It was super cool. And I would have loved to do it.

ZIERLER: A gut punch indeed.

WENNBERG: I would have loved to do it.

ZIERLER: Is there any hope of—

WENNBERG: —flying this thing?

ZIERLER: —getting the gang back together, given what its promise is?

WENNBERG: Over the years, I have said—because Falcon 9 Heavy could do it. [laughs] And Mr. Falcon 9 likes Mars a lot. So anyway, I've always said, "Hey, he should just give us a Falcon 9 launch." And we'll find the money—we'll go to all of our rich friends [laughs] and they can build the sensor.

ZIERLER: What was the timing between the disappointment of this not working out and you coming back to the Linde Center?

WENNBERG: It was pretty much simultaneous.

ZIERLER: Planetary didn't work out; time to come back to the Linde Center.

WENNBERG: Yeah. And now, it's really exciting, because anyone who has done this kind of thing realizes, after some time, that—maybe it's not everyone, but most people—most organizations just need new leadership and can really benefit from having new ideas, and new people. I had done this for so long that it really felt like now was the time. I had been trying to step down for several years, and convinced continuously to do another year, for lots of different reasons, mostly having to do with people. Who would take over? So, everything aligned really nicely now, and I'm super excited that Andy is going to take us to better and newer places.

ZIERLER: But all the way back in 2014, even for Tapio's brief time for a year and a half, what had changed, or at least what had you appreciated? What was new for you, coming back?

WENNBERG: Oh, coming back, 2014?

ZIERLER: Yeah.

WENNBERG: It was like, okay, enough of that planetary stuff. Build my lab back. New students, and that was very exciting. New students; that's always fun. We started doing more NASA mission work. After we sort of stopped doing stratospheric chemistry, we got involved in many more campaigns to look at tropospheric chemistry both from the ground and from the airplanes. We did an experiment in Mexico City on the C-130; it was an NSF experiment. That was great. We learned that—if you think about Mexico City and the pollution of the city, what we discovered was that a huge fraction of it was being driven by the burning of garbage, and by burning of the forests in the Basin. If you look at all those cars and everything else, you get focused on the cars, but then, again, if you go in and you make some measurements, you go, "Oh my goodness." Hydrogen cyanide is the major correlate of all of this, and hydrogen cyanide comes from biomass burning and from burning garbage. That was really exciting, and I think it did lead to a much more appreciation on behalf of the Mexican government of how they could improve their air quality in the city through first of all better garbage collection, but then also by changing the phasing and the timing of when they do controlled burns of the forests. These forests are mostly managed and they do a lot of controlled burning, but you can really reduce the impact of that burning by when you do it. Don't do it on windless [laughs] cold nights when it all gets swept into the city, and some of the surrounding areas.

We also then went on later to do a similar study in Seoul, Korea. Again, this was really exciting because you're making observations and measurements, in the context of people who care, deeply care, about—these are managers of air quality in the city. The Koreans had convinced themselves that the pollution in Seoul, which is a huge mega-city, was primarily a result of transport from China. Once you convince yourself of that, there's nothing to do other than yell at the Chinese. Other than say, "Sorry, people, you can't have clean air, because—China." We went and we made measurements, and it was very, very clear that a major component of their air pollution problem was domestic, related to solvents. So, not just that it was domestic, but you could really say it had to do with the manufacturing sector and the use of solvents. If you think about all the appliances that South Korea manufactures, they're painting and they're venting all of that. We're going back next year, so this will be a decade on. They have promulgated a lot of new regulations to reduce the amount of solvent use, and the evaporation of the air, and so we're going to go, at their invitation, go back to see whether or not things are much, much better now.

ZIERLER: You mentioned Mexico, Korea. What's the mode of communication for you, where, as a scientist, you make these findings, they have obvious public policy implications? Who are you talking to, and then who are they talking to, to make the necessary changes? How does that work?

WENNBERG: One of the fun things about being involved in mission science—these are all organized missions—and especially the foreign ones—they involve a lot of diplomacy. So this campaign for next year is being organized through NASA. I don't know if you know—NASA has their own diplomats.

ZIERLER: Really!

WENNBERG: Yeah. And so, you go to talk to the governments that you're hoping will be interested in having you come, and the diplomats are there to help you talk to the government. The government then gets you organized with the Environment Ministry, and so on and so forth. It ends up being the contrast to Caltech, in that these are very much high-level and then it flows down from there. So, literally it will be the Prime Minister working with NASA. Then the conversations get organized through reports, so we'll write a final report, and then we'll debrief that report. Different people will get up and say different things, and then make recommendations. So, it is done in a very collaborative way.

The Korean one was very interesting, in that the Korean government was very interested not just in having NASA come and make measurements, but in NASA helping Korea develop its own air quality monitoring program. And so, what was done was that the Korean government bought copies of most of the analytical instruments that were flying on the NASA plane. They were on one side of the plane, and the NASA instruments were on the other side—the NASA-sponsored instruments, on the other side. And, Korean scientists running, and that. So it was this big huge human resource transplant. Then the Korean instruments were put on a Korean plane, and they have been making measurements since. So, a really nice sort of human capital development through that kind of an organization.

ZIERLER: Where else in the world have you done similar kinds of work?

WENNBERG: We did a big fire study in Boise a couple of years ago called FIREX. Here, the questions were around what the smoke chemistry is, and then how does it impact air quality in downwind cities? Certainly living in California, you realize just how impactful the wildfires can be.

ZIERLER: Even all the way from Idaho.

WENNBERG: Even all the way from Idaho, yeah. But we were flying on airplanes out of Boise, and basically—it's very exciting, because you get to take the DC-8 through a fire front [laughs]—not something you normally think about being on an airplane through—and then transecting down the plume and looking at the evolution of the chemistry as you go, and then having some stations on the ground where you can see what happens. Wildfires have been, of course, increasing in the West, as the aridity has gone up. And so, in terms of air quality, generally in the United States, it has gotten better over the last 15 years, but the Western U.S. is a real anomaly, and it is being driven by wildfire, because wildfire just produces huge amounts of pollution. So, we – postdoc Lu Xu and graduate student Krystal Vasquez - did that study.

We did a ground-based study in Alabama to look at the interactions of human emissions and the forests. Because again, this is in that region where you have huge amounts of emissions from trees. And let's see, where else have we been? Most recently, we did a study as part of a project funded by NASA to do what they call tomography of the atmosphere. This was a campaign that lasted three years, and we started here in California and flew to Alaska. Then from Alaska, we flew down the Pacific to New Zealand. Then we flew from New Zealand over to—so from there, to South America. Then we went down to Antarctica. Then we went up the Atlantic. The plane basically does the going up and down from about 500 feet of the ocean, up to the ceiling of the NASA DC-8, all the way down, and then you come back around the top. So, go through Greenland and then across to Alaska, and then back down again. You do that four times – or I should say that CCE graduate student Hannah Allen and postdoc Michelle Kim do that four times - in all of the seasons. From that, you then get basically a global picture of atmospheric composition. We're still working on the analysis of all that data, but we were measuring peroxides and acids. Students would say, "Do we get frequent flyer miles?" "I think it's based on the distance between where you start and where you end. So because you ended up exactly where you started, you have zero miles." [laughs]

The IPCC In Historical Perspective

ZIERLER: I want to ask an overall question about the IPCC. Every year, there are new agreements, and the science gets better and more precise, and yet the emissions just keep going up and up and up, What does it all mean, from a historical perspective? All of these meetings, all of these promises; how do you wrap your head around the worth of the entire enterprise?

WENNBERG: I gave that lecture today. That was my final lecture in the climate class. It's interesting, because I go through the latest assessment, which was 2021—the sixth assessment—and just the highlights of what the consensus was. Basically it's bad, and not getting better. But of course one of the key things that comes out of that assessment is that, okay, we've warmed up the Earth by about 1.2 degrees since the pre-industrial times, but how much warmer the Earth gets is all about what we do now. So, you can look at this thing and say, wow, all of these assessments, what is the worth of it? Because the emissions, as you say, continue to go up. But you can also look at it—and I do—this way, and I try to explain to the students—that we have agency. We now know much, much more about how the climate is changing, because unfortunately, the signals got to be pretty big. We know a lot about what the impacts of that warming are going to be on humans, on ecosystems. And so, my own sense is that as a result of these type of activities, assessments, the science that we do, I think there is a growing awareness that this path is not good. To be honest, I used to be much more pessimistic. I've grown much more optimistic, because the economics now, we're rowing in the right direction.

ZIERLER: Renewables really do compete.

WENNBERG: They do! It's cheaper to build a solar plant than it is to operate an existing fossil fuel fired generator now. So, if you think about, say, India, which is on—per capita emissions in India are low, so they could be on the same trajectory that China was on 15 years ago, and they could just power it all with coal. But they won't, because it's a stupid thing to do.

ZIERLER: Economically.

WENNBERG: Yeah. And that's a huge change. So I think for the parts of the industrial world which is already organized around lots of expensive infrastructure that has to be depreciated, it is a real difficult problem. They're wealthy, of course, so they can choose to do something different, but you do need to abandon a lot of things that you've invested in.

ZIERLER: For the warming that is already locked in, there is of course a moral dimension to the industrialized north and all of the harms that have been created in the Global South.

WENNBERG: That's right.

ZIERLER: What is your feeling about the path forward?

WENNBERG: Again, I used to feel like, okay, we're going to have to transfer a lot of resource so that they don't do what we did, right? Because we have done harm, like you say, from a moral perspective. But you can also say all of the investments in low-carbon technologies that are now available, that are cheap, is part of that. It's part of that payment. That if you go to Africa, lots of places in Africa, what you'll notice is that everyone is talking on their cell phones. And the cell phone towers are—many of them are just powered by solar panels. And they don't have any landlines.

ZIERLER: They just skipped that whole infrastructure period.

WENNBERG: They skipped it. And why did they skip it? They skipped it because it would be stupid to do what we did, given the new technologies. And the new technologies were invented here, right? They were invented in the West. At some level then, you have—I hope, maybe—improved people's quality of life by allowing them to skip, to skip over the stupid stuff that we did. I think that's true now for a lot of the energy technologies—that Nigeria can put in solar panels at a fraction of the cost that we invested in them in getting those technologies to be scalable and cheap. I guess I'm feeling more optimistic these days. I really am. I think that things are aligning. The policies are aligning. The Inflation Reduction Act, as it was called, which was basically a climate bill, I think the topline number was just under $400 billion. But most of the subsidies were uncapped [laughs] so it's probably going to end up being more like a trillion dollars of government spending that will be matched with, say, two or three others from—this huge amount of money is going to flow into decarbonization.

ZIERLER: The topline story I'm hearing from you is that the carbon, that's a problem, but that you're optimistic because culturally, politically, and economically, we're ready to meet the moment?

WENNBERG: I think so. I do think so. And like I tell the students at the end—I had a great question. One of the students says, "Somebody told me that an individual just doesn't matter. That I could completely eliminate all my carbon emissions and it would make no difference at all." And I said, "Yeah, you've heard that story. And it turns out to be true—that if you eliminated all of your carbon emissions, it will have an infinitesimally small impact." But I said, "You might invent the new battery technology. You might find yourself on an airplane next to a senator and get them to think about something. You might end up being Greta Thunberg, who inspires thousands of people your age to care about something." And so, you're really just trying to get people to understand that this is not hopeless, and that they and all of us are going to be the ones who make a choice about what that future looks like. Based on the stuff we've learned over the last 50 years, I'd say we don't want to go to the bad places that are predicted on our current trajectory, and I think people are realizing that, so I guess I'm feeling a little more optimistic.

ZIERLER: You talked about carbon. What about air pollution? Where are things headed on a global scale? Do you feel good there as well?

WENNBERG: Generally, yeah. There's an interesting problem, which is that people like to talk about co-benefits, right? And this is one where it's probably anti-benefits. In other words, the Earth has warmed about 1.1 or 1.2 degrees. It would have warmed probably another four tenths of a degree had we not polluted it. So, even today, you can look outside and there's a lot of haze, and that haze is driven by burning stuff, mostly.

ZIERLER: Including fossil fuels.

WENNBERG: Yeah. A thing that reduces visibility also reflects away sunlight, and it also changes the way the clouds behave. Together those lead to a significant cooling. But people don't like polluted air, for good reasons. So now, we're on this trajectory where we're really cleaning up. Of course the United States was one of the first where we had the Clean Air Act, but it's being followed across the world. China's air is getting cleaner, fast. As a result, we're going to get more warming. It'll be a temporary forcing. It'll be short-lived, because it's only—this is three-tenths or four-tenths of a degree that we're going to get as we clean up the aerosol pollution. But it is an additional heat source that we're going to see in the next couple years. We're already seeing it. Generally though, I'd say electrification is good. It really is good. It's good for climate, and it's good for air quality. The more we can stop burning stuff, the better. Electrification of everything, as they say, is also a way to get better, cleaner air. And we're doing that. So, I feel optimistic.

The Linde Center Legacy and its Future

ZIERLER: To round out our conversation—I think we can do it!—the two things to touch on are, looking back on your directorship of the Linde Center, what are you most satisfied with, at a very general level, in terms of the Center, in terms of your directorship?

WENNBERG: Hiring great people. Absolutely top number one. Hiring and getting them started on their fantastic careers. Nothing better than to be able to have new colleagues who are just amazing, and to be able to help them with their science, and getting them started.

ZIERLER: And there's an internal momentum there, where there's a generational value here.

WENNBERG: Oh, yes.

ZIERLER: That tomorrow's junior faculty are coming because of—

WENNBERG: Yeah, that's by far the most satisfying thing. What was the second question?

ZIERLER: The science. Now that you've stepped down, what's most important to you as you look ahead?

WENNBERG: Hopefully this interview won't get published too soon, because—

ZIERLER: That's your choice.

WENNBERG: Yeah. I'm in the middle of thinking about a new career, doing program management for philanthropic organizations—around climate.

ZIERLER: Oh, wow.

WENNBERG: The theory of my next few years is that sometime in the next two or three years, I'll take a sabbatical, and I'll go work in a foundation, or at some associated non-governmental organization, and try to learn the trade, and see if it's something I really want to do. Then I would try to do something like that half-time, for three or four years. And then I would step down, from Caltech. I'm early sixties, and so I think you always look around, and you say, "Either I'm going to just continue to do this," and you look at people who—I don't know of any Caltech people who retire, really. [laughs] It's not—

ZIERLER: No. There's no such thing.

WENNBERG: Yeah. But I do—

ZIERLER: There are administrative distinctions, but that doesn't mean you don't work.

WENNBERG: Exactly. The question is, if you're going to keep working, what do you want to do? At the moment, the last few years—I think the pandemic also influenced this—was, I sort of feel like I have another act, but maybe it's not what I'm doing right now.

ZIERLER: Do you think—people talk about this all the time—are there diminishing returns in a scientific life after a certain age? Obviously it's individualized on your own experience, but in broad measure, do you think that if you were to remain full-time in scientific inquiry, you're just not capable of doing what you might have been able to do 20 or 30 years ago?

WENNBERG: Probably. I think it's probably true. Who knows, though, because so much of what we do is through others—our students, postdocs, staff. The question I guess you're asking is, can you inspire really, really new and creative people, as well. I think you can. I think there are lots of examples of that. But the part of it that is like my own science and what I'm actually doing, I do feel like it's a bit diminishing. I don't know if it's my own capabilities or my own lack of creativity, but I sort of feel like—I'd like to try something else, and I'd like to do it while I'm young enough that I can imagine doing something really different, and do it.

ZIERLER: Last question. For all the ways that you've been involved in policy, either in the periphery or in those moments when you really are, what have you learned about yourself, where you think, "I want to drill down into this skill that I didn't know I had"? How might you operationalize that for this next act?

WENNBERG: Well, I've always—I do think that I'm curious, about things, and unlike a lot of others, I really do like to get into the weeds of the economics of things. Because in the end, I think it's so profoundly important as to whether something is going to end up impactful. I'd say that's one of the things that I'm really thinking about, going forward, is developing new skills in that area, and being able to—one of the things I've noticed—this is not so much in policy, but it's in advising—is that particularly in small foundations, there's a lot of stupid stuff that is being funded. I think it reflects just humanity, right? You've got someone who has money but not a huge amount of money, and they meet so-and-so on the golf course, and the person on the golf course tells them about this great thing they're doing, and a check gets written. And so, I think there's just a role to be played here.

ZIERLER: To be a hard-nosed scientist in a policy kind of environment.

WENNBERG: Yes, and to have a really good bullshit detector. Just to be able to help these folks ask the right questions. "Before you do that, here are the things that I think you should think about and ask." To know whether this is a useful thing to do. So that's what I've been thinking about.

ZIERLER: Something to be excited about in the future for you.

WENNBERG: Yeah.

ZIERLER: Paul, I want to thank you for spending all this time with me. I really appreciate it.

WENNBERG: Thank you, David. It has been fun.

[END]