Susan Solomon: at the frontline of research on the ozone layer and its role in Earth’s climate

By Dawn Stover | May 22, 2018


On May 24, the Royal Swedish Academy of Sciences will award one of the world’s largest scientific prizes to two of the world’s top climate researchers. In the presence of Swedish royalty, MIT professor of atmospheric chemistry Susan Solomon and Princeton University senior meteorologist Syukuro Manabe will receive the 2018 Crafoord Prize in Geosciences—and 6 million kronor, equivalent to about $690,000—for “fundamental contributions to understanding the role of atmospheric trace gases in Earth’s climate system.”

The Crafoord Prize is an annual award established by Swedish industrialist Holger Crafoord and his wife Anna-Greta Crafoord. The Academy is responsible for selecting the Crafoord Laureates, and the prize is awarded in disciplines that complement those for which Nobel Prizes are awarded.

Manabe, one of the founders of climate modeling, did groundbreaking work predicting how the climate system would respond to increases in carbon dioxide. Solomon, who is on the Bulletin’s Science and Security Board, is best known for solving the mystery of the ozone hole, the annual thinning of the protective stratospheric ozone layer over Antarctica.

Solomon’s studies and measurements in Antarctica identified both the mechanism for ozone destruction and the culprit: chemicals called chlorofluorocarbons, which were once widely used as refrigerants and aerosol propellants. Identifying the problem led to a global solution called the Montreal Protocol, an international treaty to phase out ozone-destroying gases and replace them with less harmful chemicals.

Although Solomon’s recent studies have shown that the ozone hole has begun to heal, there is still much to be learned about how the ozone layer is responding to human intervention, and about what can be done to improve its recovery during what Solomon calls the “accountability” period of ozone research. (A group of scientists reported last week that emissions of a banned chemical appear to be rising, which suggests that someone is cheating on the agreement.)

Solomon, who has co-chaired a working group of the Intergovernmental Panel on Climate Change, is also studying how changes in the stratosphere affect Earth’s winds and climate systems. Bulletin contributing editor Dawn Stover spoke with her about her work and the Crafoord Prize.

BulletinHow did you learn that you’d won the Crafoord Prize? Did you get one of those 5:00 a.m. calls from Stockholm?

Solomon: I was actually on a ship in the middle of the Antarctic Ocean when they called me. There was no telephone, and we had extremely limited Internet, so they sent me an email which I didn’t see for a day and a half.

BulletinWhat were you doing in the Antarctic?

Solomon: I wish I could tell you that it was a research cruise. I did my real research in the Antarctic in the late ’80s and early ’90s, and all of our work was out of McMurdo Station, which is on land. But I’ve on a couple of occasions been a lecturer onboard a tourism ship, and that’s what I was doing.

BulletinSo you were having fun.

Solomon: I was doing a little outreach, and talking about how wonderful the Antarctic ecosystem is, and all the great creatures that live down there, and how climate change is affecting some of them. So yeah, it was a lot of fun.

BulletinHow many times have you been to Antarctica?

Solomon: Seven times.

BulletinSo you won the prize for solving the mystery of what was causing a hole to appear in the ozone layer. Spoiler alert for our readers: The chlorofluorocarbons did it. For the benefit of people who may not be familiar with your work, can you explain how you figured that out?

Solomon: Sure. Back in the 1970s people first started wondering whether using chlorofluorocarbons might someday deplete the ozone layer, but we thought that the effect would be a few percent, and it would be far in the future. No one ever expected to see such a big depletion, and in 1985. So it was a tremendous shock when the British Antarctic Survey announced that they’d been measuring ozone since the 1950s; it had spent 20 years being constant and then, boom! It just kind of fell off the map. Nowadays there’s only about half as much ozone over your head when you’re in the Antarctic in October or November as there would have been in, say, the 1960s. And we knew that similar things were not happening in the Arctic. So if this was caused by chlorofluorocarbon chemistry, it had to be a chemistry operating in a very different way than what we were expecting. And that turned out to be what it is.

BulletinHow did you uncover the chemistry?

Solomon: I started wondering about the fact that Antarctica is the coldest place on Earth, which means that clouds form in the Antarctic stratosphere during the cold season. In the United States, you never see clouds at stratospheric altitudes. You see them in the lower atmosphere, but not up in the stratosphere where the ozone layer exists. And I thought that those clouds might be changing the chemistry in a simple but pretty fundamental way: If the chlorine from the chlorofluorocarbons was reacting on the surfaces of those stratospheric clouds, that might explain why the Antarctic was so different from the rest of the world. I looked at what sorts of reactions might be happening on those surfaces and proposed that particular types of chemistry might be accelerated by those clouds, and that turned out to be right.


Solomon: I just want to emphasize one thing.


Solomon: It’s not that the clouds themselves are causing the ozone hole. There would be no ozone hole if we hadn’t put chlorofluorocarbons in the atmosphere, but the clouds are basically enhancing [the chlorofluorocarbons’] ability to be effective, particularly in the polar regions. We now know that we get some of the same chemistry happening in the Arctic, not to nearly the same degree, but a significant amount of Arctic ozone depletion can also happen, and even over middle latitudes ozone depletion is worse than we expected. So it’s an interesting example of an environmental issue where we made some predictions and they turned out to be very conservative, actually. The problem is much more severe than what we thought.

BulletinI think it was the year 2000 that the ozone hole reached its largest size, is that right?

Solomon: You can actually use all kinds of different ways to define “largest.” You could talk about the depth of the ozone hole, in other words, what percent of the ozone that used to be there has been destroyed. If you do that, then you’re correct, the deepest ozone holes happened around the year 2000.

BulletinThat sounds a bit like the melting of the icecaps; it’s not just how much area opens up, but also how thin the ice is.

Solomon: Exactly.

BulletinIt wasn’t until 2016, though, that a team that you led identified what I think you said were the first signs of healing of the hole.

Solomon: That’s right.

BulletinIs the ozone layer still at risk, or will the hole keep shrinking from now on?

Solomon: We have every reason to believe that the ozone hole will recover. It’s really quite a remarkable science-policy success story. As I showed in the 2016 paper, if you analyze the data carefully and look at the month of September in particular, you can show that the rate at which the ozone hole opens up every year is getting slower and slower, which is exactly what we expect from the fact that we’ve stopped putting the chlorofluorocarbons into the atmosphere, but those molecules have very long lifetimes. They live in the atmosphere anywhere between 50 and 100, even 500 years, depending on which one of the molecules you talk about, and there’s many different ones that were used in applications like spray cans, solvents, foam blowing, and refrigeration, of course. Since we stopped emitting them in the late 1990s, what you see today are the leftovers decaying away slowly following a very long lifetime. There will still be ozone holes probably as late as 2050, but probably by the time you get to 2060 or 2070 they won’t be happening anymore except maybe once in ten years or something like that; and you might even have a year in the 2030s or 2040s when you might not have an ozone hole. That will be an incredible year. What we showed, which is really exciting, is that, if you account for the kind of variability that is happening and you account for how chlorine has gone away, not only is the rate at which the ozone hole opens up getting slower, but also the area of the ozone hole in September is shrinking and the hole is getting less deep.

BulletinThat’s good news. But a study published recently reported that, although the upper stratospheric layer of ozone appears to be recovering nicely, the ozone in the lower stratosphere is still declining.

Solomon: Not in the Antarctic. There it’s very clear that ozone is beginning to recover. But in the tropics, there’s some evidence for ozone losses in the lower stratosphere that are being discussed and debated right now. I think it’s too early to say whether that’s just interannual variability or whether perhaps the way that the different satellite data steps have been stitched together to produce that result might not be quite right. I think the tropical issue is a new and very interesting science area for the community to explore, but the Antarctic is clearly recovering and it’s a very different situation.

Bulletin: Are you still working on those new studies, or is your ozone work now done?

Solomon: No, it’s not done. I’m having a ball with the recovery. I find the issue of how the ozone hole is displaying its healing process to us to be absolutely fascinating, so I’m having tremendous fun going after that problem, and I have continued to do a whole series of studies on it that look at different pieces of that issue.

Bulletin: Antarctica is where you started getting the data confirming your theory about the ozone hole. What do you remember most about your time there?

Solomon: Oh, it was just so wonderful. It is such an incredibly beautiful, awe-inspiring, spectacular place with the most remarkable wildlife that you can imagine, although at McMurdo there’s not a lot of that wildlife. When you do see something like a group of emperor penguins, it’s really impressive, and whales, and seals, and everything else that’s down there, but also just the remoteness of the place. It really is like going to another planet. From a scientific point of view, it was exploration at its best. We had this tremendous mystery, and very few measurements had been made there. If the ozone hole had popped up at another latitude, we would’ve had a lot more information to go on, but it was just in every way the unexplored last continent.

BulletinAfter your big breakthrough with the hole, how did you move on from that?

Solomon: I spent many years, and still do, just doing chemistry of the stratosphere because I find it interesting. I have started doing more work in climate, including this issue of whether changes in wind patterns in the stratosphere actually influence lower altitudes. When we talk about changes in the global climate, nowadays we’re looking at warming, relative to 1880, of something like 0.8 degrees Celsius, or on the order of 1.5 degrees Fahrenheit. But when you talk about the stratosphere, the cooling that’s occurred due to the ozone hole is just massive. I mean, it’s on the order of five degrees per decade over a few decades, so you’re talking about 10 degrees, 12 degrees Celsius. The reason for that is that ozone provides the source of heat to the stratosphere. If you take all the ozone away, which is what the ozone hole does, then there’s no heat and it just cools like crazy. The potential of that to modify the wind system is very clear, and then to look at how that might propagate into the troposphere all the way down to the ground became something that my colleague, Dave Thompson, and I got interested in, and we did a series of papers, and still are continuing to work on that issue. And it turns out to be pretty important.

BulletinHow does that affect the winds?

Solomon: As the stratosphere has cooled, it’s changed the winds in such a way that the troposphere has also cooled over the surface of the continent, and it has strengthened the polar vortex. You often hear how cold weather is caused by the polar vortex here in the United States. The same thing happens in the southern hemisphere to some degree. But in the case of the ozone hole, the polar vortex got even colder and even more stable, and therefore kind of shrunk over the continent. So it made places like the South Pole itself, and the high plateau of East Antarctica, colder than they would have been, and it made the regions just outside the polar vortex warmer—because they’re just not getting those excursions of really cold air from higher latitudes anymore, the way they used to. It has also affected rainfall patterns: If you shift the wind patterns, you can shift where the humidity and the rainfall is going. We’ve shown, and other people have too, that that is actually pretty important for the climates of places like South America, Australia, even parts of Africa. The ozone hole has really rocked the southern hemisphere, as far as climate goes.

BulletinIn your work on climate, I imagine that maybe you’ve had some encounters with climate deniers, but I don’t recall ever hearing of any ozone hole deniers. Is there something about the science of climate change that makes it more controversial?

Solomon: There were ozone hole deniers, actually, in the very beginning. There are going to be some people who will distrust the science for whatever reason. As the science gets stronger and clearer, it becomes more and more difficult for that kind of view to be tenable, and I think one of the things that the ozone hole always had going for it was the massiveness of the effect. When you look at ozone measurements from the 1950s and ’60s, and you see values that are twice what they are today, it’s just a lot easier for people to understand. You don’t have to be a statistician to understand a change of a factor of two. But when you talk about climate, I think it’s much more different for people to wrap their heads around. Another thing about the ozone issue is that it was a very clear-cut area as far as what the human impacts would be. If there were to be significant ozone depletion, then it would cause issues like skin cancer and cataracts. Everyone knows that ultraviolet light is bad for you, and if you’ve ever been sunburned, you know it’s not something you want to do too often because it can lead to skin cancer. So the degree of personal concern was universal, whereas when you talk about climate change, I think those of us particularly in the developed world, we feel pretty safe. We have a society that’s rich enough to insulate itself against the slings and arrows of climate fortunes. We’ve got years of bad harvest and years of good harvest, and it doesn’t lead to famines in the United States. We just don’t feel it as a threat in the same way that we understand skin cancer as a threat. And finally, of course, the solutions to the [ozone] problem were not that difficult to identify. In the case of climate change, a lot of people imagine that’s there’s no way we could ever get off of fossil fuels. But I think it’s been shown, more and more now, how practical many of the solutions actually are. In the case of the ozone issue, the initial problem really had to do with the use of chlorofluorocarbons in spray cans, and a lot of those spray cans were for things like hairspray and deodorant. That was not all that difficult to shift away from, and we still have hairspray; it just doesn’t have chlorofluorocarbons in it.

BulletinIt is interesting how fixing the problem took a combination of policy (in the form of a global treaty) and a switch to using different products. Would that same combination work for global warming as well?

Solomon: I think there is some degree of analogy. What the ozone issue shows you is that it is possible for society to come together and make a decision to regulate a chemical internationally. Every country in the world has signed on to the Montreal Protocol to protect the ozone layer. It’s really quite amazing how chemical plants in places like China and India have been closed along with chemical plants in this country. That’s a remarkable scientific and policy success story, but it’s also dealing with an industry that, all told, is something like only 15 different chemical companies worldwide. I don’t know the exact dollar value of the chlorofluorocarbon industry at its height, but it wasn’t anything even remotely resembling the fossil fuel industry. Everything we do relies on energy, and as long as energy relies on fossil fuels and it’s the engine of our economy and the economies of all other nations too, that makes it difficult for people to contemplate the idea of being part of an international agreement to regulate it.

BulletinNot to mention industry resistance.

Solomon: I must say, I give the chemical industry some credit. Some of my colleagues argue that they were difficult to deal with in the beginning, and I saw that occasionally, but I also think they were willing to change. The chemical companies are all about innovation. They are always looking for different molecules to make, and I don’t think they shrank from the challenge of doing something different.

BulletinHow much of that comes down to leadership at the top?

Solomon: Well, leadership’s important, but what’s also important is what the nature of the industry actually is. An industry whose nature is to make stuff, I think, honestly, that they’re going to be different than an industry whose nature is to extract stuff.

BulletinWhen you’re thinking about the impacts of climate change that are still mostly off in the future, what worries you most about it?

Solomon: The impact on the developing world. The number of people who will be suffering in countries that contributed very, very little to the problem and just don’t have the resources to deal with the impacts. I’ve done a lot of work recently on issues like the potential effects of climate change on food production in places like Africa, and it’s very concerning to me that, as we move into the rest of the 21st century, I think it’s going to be very hard to grow food in some parts of Africa. Again, we’re insulated because we’re rich enough to afford things like irrigation. A fair amount of Africa doesn’t have that, so they are completely reliant on what falls from the sky as rain. If that’s changing, or if it’s hotter and there’s more evaporation, what’s that doing to them? Certainly ecosystems and agriculture, as well as human health, are at risk. People die from heat waves. People died last year in India, if you remember.

BulletinYou got my attention with a paper that you edited last year that talked about the risk of a planet so hot that literally billions of people could die without air conditioning.

Solomon: Beyond a certain temperature, the human body is unable to cool itself if the relative humidity is too high.

BulletinSo you can’t live in that place anymore?

Solomon: Well, you can only live in that place if you’re living in a bubble, right? I mean, kids can’t go outside to play. You’ll have to be in air conditioning essentially all the time. Maybe you can quickly go from your vehicle to your air-conditioned store or home or whatever, but you’re going to be living in a surreal kind of world.

BulletinYou’ve been on a lot of advisory panels, including here on the Bulletin‘s own Science and Security Board, and you’re very well-known as a scientist. You get inquiries from journalists like me all the time, and I’m wondering how you balance your public life as an ambassador for science with your professional and private life as somebody who is still actively doing research.

Solomon: I view it as a responsibility to do some public outreach, and I’m particularly aware of the fact that there’s a fair number of younger, early-career women in this field, but I started out at a time when women were extremely few and far between. Because of that, I put effort into activities that I think help with the role-model aspect. I still have to limit it. Sometimes I have to decide, “No, I can’t do that because I just don’t have time and it would take too much time away from the research that I love.”

BulletinIn another interview, you talked some about the dangers of scientists becoming too sure of themselves as they get older, and the importance of examining your own thinking and staying open to completely new ideas. In the context of climate change, how do you advocate for that kind of “uncertainty” without playing into the hands of people who are promoting uncertainty as a reason to do nothing about global warming?

Solomon: Well, don’t ever forget that uncertainties cut both ways. It’s also quite possible to underestimate how bad things could be, and we certainly did that with the ozone problem. Climate is all about risk. There’s a risk that we’re going to be over-careful, I suppose, in doing things to protect ourselves from climate change. But there’s a much bigger risk that we’re going to be under-careful and not do enough. We don’t buy fire insurance for our homes because we think the home is going to burn; we buy it because of how bad it would be if it actually did. In this case, we know that our home is going to get a lot hotter, and it could happen faster than we think. I see so many indicators that a lot of things are going to be bad. An easy example is wildfire: We’re seeing more and more of it, and the fire season is now longer than it used to be. We don’t really understand ecosystem dynamics well enough to fully predict how bad it could get, but I think we have every reason to believe that it’s going to be just horrible. There will be places where the landscape will be permanently altered. I find that sort of thing to be heart-wrenching, because of what it’s going to do to the people who live in those places.

BulletinYou seem to really love a challenge, so what is the encore to solving the ozone puzzle?

Solomon: I think we are in an interesting phase now with ozone that some people call the accountability period. The first period of ozone research sometimes is called credibility: Was [the ozone hole] real? Then we moved into manageability: What can we do about it? Now we’re into accountability: Have we done all the things we could do? There are still, for example, chlorofluorocarbons out there in what’s sometimes called the banks, things like old refrigerators and old air conditioners. To what extent is our recycling process operating at top efficiency, to better recover all that material that we don’t need anymore? I think that’s a very, very interesting question. And then, just understanding the chemistry of how the ozone layer responds as chlorine decreases is a fascinating and interesting problem, too. I’m not bored with ozone yet.


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