Interview with Susan Solomon


Dr Taba recounts:


On Tuesday, 14 March 2000, our interviewee in this issue, Susan Solomon, smiled proudly as President Clinton placed the Medal of Science award about her neck during a ceremony in the White House in Washington DC. Susan was one of 12 scientists selected and one of the three award winners in chemistry. Congressman Mark Udal delivered the following message:


Washington DC, 14 March 2000 — Susan Solomon receives the National Medal of Science from President Clinton

... Dr Susan Solomon ... is the recipient of the 1999 National Medal of Science. Dr Solomon is a senior scientist at the National Oceanic and Atmospheric Administration, based in Boulder, Colorado, and is the first NOAA scientist to be awarded the medal, which is the nation’s highest scientific honour. She is also the recipient of many other honours and awards that recognize her important work.

In commending her accomplishments, Secretary of Commerce, William Daley, called Dr Solomon “one of the most important and influential researchers in atmospheric science during the past 15 years”.

Dr Solomon first theorized in the 1980s that the explanation for the Antarctic ozone hole involved reactions on polar stratospheric clouds (PSCs), not just gas molecule reactions. These reactions release chlorine molecules, which separate and act as catalysts in destroying ozone. Dr Solomon confirmed her theories with solid data observed during two expeditions to the Antarctic in 1986 and 1987, when she found evidence for enhanced reactive chlorine. Because of her discoveries, scientists were able to understand how the ozone hole is linked to chlorofluorocarbons (CFCs) and other man-made compounds.

Dr Solomon and other leaders in her field provide important role models for today’s students as they prepare to meet the demands of tomorrow’s technology-based economy. But it is not only the young who can benefit from Dr Solomon’s example. She cites as the most important lesson from her research the “need to keep an open mind on environmental issues”.

Susan was born in Chicago on 19 January 1956. Her mother was a fourth-grade teacher and her father sold insurance. Her brother is also a fourth-grade teacher. Her interest in science, especially the natural world, began when she was less than 10 years old. She was fascinated by the work of Jacques Cousteau and decided to become a marine biologist and study whales.

Her discovery of chemistry was a revelation and a fascination. In high school, she won a third-place prize in an international science fair. Interestingly enough, her project was the use of light to determine the percentage of oxygen. She went on to study chemistry at the Illinois Institute of Technology (IIT) in Chicago and spent her third year in France as part of an exchange programme, before returning to IIT.

Susan wanted to become a scientist at the Ph.D. level and do professional research. But she also knew she did not want to spend the rest of her life studying chemistry that was only important in a test tube. In her senior year at IIT, she discovered atmospheric chemistry and this opened her eyes to the fact that one could actually engage in chemistry that actually took place on the planet. Susan chose the University of California at Berkeley for her graduate studies so that she could work with Harold Johnson. Harold was well known for his work in the early 1970s on the detrimental effect of supersonic transport on the ozone layer. So she found the ideal environment where she could do chemistry in the ocean of air.

To discover more about Susan’s brilliant career, readers are invited to study the following pages. Some of the professional positions she has held are: Research Scientist, Programme Leader and Senior Scientist at the Aeronomy Laboratory of NOAA, Boulder, (1981 to the present); Affiliate Scientist at the National Center for Atmospheric Research (NCAR), Boulder; Adjoint Professor, Department of the Astrophysical University of Colorado, Boulder; and, perhaps most importantly, Head Project Scientist, National Ozone Expedition, McMurdo Station, Antarctic (August-November 1986 and August-November 1987). Susan has to her credit a list of more than 20 scientific honours. These include: James B. MacElmane Award of the American Geophysical Union (1985); Gold medal for exceptional service, US Department of Commerce (1989); Henry G. Houghton Award for excellence in research, American Meteorological Society (1991); Member of the US National Academy of Science (1992); Scientist of the Year, R & D Magazine (1992); Honorary Doctorate, University of Colorado (1993); Arthur S. Flemming Award for excellence in government service (1994); Honorary Doctorate, Tulane University, New Orleans (1994); H.J. Reid Award, NASA Langley Research Centre (1994); Foreign Associate Académie des Sciences de France (1995); Honorary Doctorate, Williams College, Williamston (1996); Stratospheric Ozone Protection Award of the United Nations Environment Programme (1997); Co-recipient, Climate Protection Award of the Environmental Protection Agency (1998); Carl-Gustaf Rossby Research Medal, American Meteorological Society (2000) and a Member of the Academia Europea (2000). She is a Fellow of the American Geophysical Union, the American Meteorological Society, the Royal Meteorological Society and the American Chemical Society. If I have counted correctly, Susan is, or has been, on more than 40 editorial boards, committees and science teams. She is the author or co-author of some 150 scientific papers in journals and other publications, including the article entitled “On the role of the weather in the death of R.F. Scott and his companions” and a forthcoming book called “The coldest march”. Some topographical features in the Antarctic carry her name, such as “Solomon Glacier” and “Solomon Saddle”. In my opinion, this is the highest honour Susan has received.

This interview has a special significance for me. I took up my employment in the WMO Secretariat in October 1960. The year 2000 marks my 40th year of association with WMO. My first WMO Bulletin interview was published in January 1981, that is to say 20 years ago. This is the 80th interview I have conducted and, at 44, Susan is my youngest interviewee.

This interview took place in Prague in June 2000, during the annual meeting of the European Academy.

H.T. — How did you get to go to NCAR?

S.S. — I received a University Cooperation for Atmospheric Research (UCAR) graduate fellowship in 1977. The programme included working at NCAR every summer for two years. I was fortunate to work with Paul Crutzen1, who was Director of the Atmospheric Chemistry Division (ACD), and Jack Fishman, who is also a well-known and capable researcher. In those days, data were not digitized. I tried to evaluate as many different stations as possible that had measurements of the vertical profile of ozone in the northern and southern hemispheres and in the tropics in order to establish if there were any differences. The resulting paper was the first scientific paper that I participated in and is still quite widely cited today. I can not really take any credit for this work except as a human calculator!

H.T. — When did you receive your Ph.D?

S.S. — I spent two years at Berkeley doing the required course work. This included quantum and statistical mechanics and kinetics, which is related to, but far from the heart of, atmospheric chemistry. When I found out about another student programme at NCAR as a graduate assistant, I jumped at the chance. Since Harold Johnson and Paul Crutzen were good friends, I was able to make an arrangement which allowed me to do my thesis work at NCAR rather than at Berkeley. Harold was extremely supportive. I got my doctorate and then a job at the NOAA Aeronomy Laboratory.

H.T. — Let’s talk about your post Ph.D. work.

S.S. — The four years after receiving my Ph.D. were busy. A major focus was my work with Rolando García, at the ACD. For some 20 years we have been developing a two-dimensional stratospheric model to look at a broad range of chemical and physical processes that influence ozone. We were the first to implement a framework for realistically describing stratospheric chemical transport in two dimensions. This is important not only for ozone but also for a broad range of other species. Looking at latitudinal gradients of species such as methane, we could see that the transport was much more realistic. We have come a long way since then in further developing and applying the model. We wrote half a dozen papers where we examined things in a way no one had ever done before in stratospheric chemistry. The major credit goes to Rolando in the sense that the circulation was formulated in an innovative way.

H.T. — Can we talk about the discovery of the ozone hole?

S.S. — The depletion of Antarctic ozone was revealed by a British team in May 1985. I concluded that it had to be right but, whether the depletion was due to chlorine or a natural process such as cosmic rays, was a question requiring a great deal of work. The British had started making measurements in the late 1950s and for about 20 years, the ozone situation was normal. In the late 1970s, however, it began dropping. By 1984, it was some 30 per cent below what it had been during the austral spring in the historical record, way below the natural variability. Shortly after the publication of the British paper, a scientist at NASA suggested the depletion might be due to the oxides of nitrogen (NOx) produced in association with the solar maximum coming down from the thermosphere into the stratosphere. This was ironic, because my Ph.D. thesis had been on that very topic and I had worked with Ray Roble and Paul Crutzen at NCAR trying to quantify the whole issue of transport of NOx from the thermosphere to the stratosphere. I was soon convinced that that could not be the cause of the ozone hole. Instead, I took all the Antarctic ozone-sonde data that had ever been taken and found that the ozone depletion was happening low down, near 20 km. The auroral NOx mechanism would produce greater ozone losses much higher up, near 40 km, so it did not have much potential to change the shape of the profile in the observed way. A third idea was of a systematic change in atmospheric dynamics which reduced the ozone dramatically.

H.T. — What did you think was the culprit?

S.S. — I think it is fair to say I have done more than anyone else to show how surface chemistry can affect ozone. By surface chemistry, I mean reactions between gas molecules and solid or liquid surfaces. It is akin to the idea of catalysis when processes occur on a surface that do not occur in the gas phase or occur much more slowly. The Antarctic is the coldest place on Earth. That’s why clouds can form in the stratosphere there. What I suggested was that the reaction between hydrochloric acid and chlorine nitrate might take place on the surfaces of PSCs, thereby changing chlorine from a form which is not ozone-damaging to other chemical forms which are. This was my suggestion and it has indeed turned out to be the initial key reaction in producing the ozone hole.

H.T. — Tell us now about the field programme.

S.S. — A close colleague at the Aeronomy Laboratory, John Noxon, had been doing field measurements for many years. He made visual absorption spectroscopy measurements using sunlight or moonlight and looking for the weak absorption due to nitrogen dioxide, ozone and other molecules. He died shortly before the British paper on the ozone hole was published but left behind a rich Aeronomy Laboratory tradition in that kind of measurement. He also left behind an instrument which he and his colleague, Art Schmeltekopf, had been developing for several years, with the intention of measuring nitrogen dioxide and ozone. In March 1986, we had a meeting in Boulder on global ozone issues. We set up a session to discuss the Antarctic and see what we could do. That was the first meeting where the idea of surface chemistry on PSCs was presented. We also talked about the solar theory and dynamic theory. The question was raised as to what could be done in the way of taking measurements that would show what was going on. Many people were still saying that the British measurement was maybe just wrong. So it was important to see if the ozone hole really existed.

H.T. — What happened next?

S.S. — We knew from the British data that there was an ozone hole in October and not in February: the ozone hole opened between August and October. This meant that the earliest opportunity to go was at the end of August, when the winter flights went in. We then discussed instruments and measuring nitrogen dioxide and ozone using visual absorption spectroscopy, as had been done by the Aeronomy Laboratory. John was dead and Art was committed elsewhere, so I said I would go. At first, everybody laughed, because I was a theoretician, but there was nobody else and the worst that could happen would be that I would get no data. A few months later, 16 of us from four institutions were on our way to the Antarctic. Ours was the first group of scientists allowed to go and that was thanks to the efforts of the National Science Foundation and especially John Lynch. I was chosen as the group leader, probably because I was willing to talk to the media.

H.T. — I assume that your field programme was a success and you brought back a great deal of data and observations.

S.S. — We measured not only nitrogen dioxide, but also chlorine dioxide. We made the first measurements of active chlorine and showed that there was about 100 times more of it than can be explained by gas-phase chemistry and therefore it had to come from heterogeneous chemistry. That was the explanation for the ozone hole. I am delighted that those measurements were the first to show that chlorine is enhanced by PSCs but I would like to stress that I worked with an instrument that I had neither designed nor built. My contribution was the insight to make the measurements that showed that we had chlorine dioxide. Chlorine dioxide is a molecule that breaks down rapidly in sunlight (photolysis), so we thought we would see it at only night, using the moon as a light source.

Susan Solomon in her office at the NOAA Aeronomy Laboratory in Boulder, Colorado

It was exciting, standing on a roof at -40°C and 65 km h-1 winds, holding up a mirror. Like all absorption measurements, we do the measurements relative to a background spectrum. One day in the middle of September, I looked up and saw the Sun for the first time since the end of August. I realized that it would make a perfect background for the moon data. So I ran back to the laboratory and took some data using the direct Sun as a light source. Those data turned out to give a signal three times better when used as the background for the lunar data..

H.T. — What did you do when you returned?

S.S. — In November 1986, we left the ice with a lot of data to analyse but convinced that we had seen chlorine dioxide with the moon. We were equally convinced that nitrogen dioxide was not the cause of the ozone hole. We had also measured the ozone depletion. Four different instruments were able to document clearly that the ozone was normal at the end of August and then dropped systematically so that, by the end of September, there was only about two-thirds as much as there had been at the end of August. For several months, I steeped myself in data analysis. I had a small epiphany when I found a way to separate the chlorine dioxide signal from our daytime data. We now had the full diurnal cycle—daytime and night-time—and, in both cases, it was 100 times more than what it should have been based on gas-phase chemistry. Many different things were all pointing to chlorine dioxide. I wrote a paper and submitted it for publication in January 1987.

H.T.     How were your interactions with the media when you returned home?

S.S. — As soon as I got home from the Antarctic, the media pressure was intense. We released one press statement from the Antarctic and when I read it now,10 years later, I think we played it exactly right. We said with confidence that there was not much nitrogen dioxide, that ozone was depleted in a remarkable way between 15 and 20 km. We did not talk about chlorine dioxide or monoxide until our papers were accepted for publication. When you are involved in scientific work in the public eye, it is easy to be carried away under the pressure to be quick. In most cases, a delay of a few months or even a year does not make any societal impact but it does make the work solid and more credible. I was barraged by phone calls. I even got one while I was in the Antarctic from a famous journalist. In order to do that he had to claim that he was a member of my family and that there was an emergency. Fortunately, I did not accept the call. I do not think the world would have been better served by our having made more statements earlier, or by my giving this person a scoop.

H.T. — In 1992, some six years after your return from the Antarctic, you were elected to the US National Academy of Sciences and later on to the French Academy of Sciences. How did you feel about it?

S.S. — It was unexpected and I was thrilled and honoured. I was so deeply immersed in my scientific work that I had no clue as to what its ultimate impact on my personal career might be. I did not think of myself as "young" or "a woman". When I walk into a meeting, for example, I am not going to notice if I am the only woman in the room or if I am the only person in his or her early 40s. Becoming a foreign associate of the French Academy also has a special significance because of the time I spent in France and my affection for the country. The French Academy is one of the oldest in the world and has a distinguished scientific history. It is a great pleasure to be part of it.

H.T. — Crutzen, Rowland and Molina received the first Nobel Prize ever for atmospheric chemistry. What would you like to say about it?

S.S. — The Nobel Committee wisely chose the three most qualified people for the Prize. (By the way, Alfred Nobel’s will stipulates that the Prize is limited to no more than three recipients,) I went to Stockholm to take part in the ceremony as Paul Crutzen’s guest. It was great to watch Paul, Mario and Sherry, who are all good friends, receive the Prize. Molina and Rowland were the first to point out that gas-phase chemistry could cause ozone depletion, that CFCs have lifetimes in the atmosphere of 50 to 100 even 500 years, depending on the molecule, and that any atmospheric change they produce would last a long time. Crutzen also made unprecedented contributions to our understanding, particularly with regard to NOx chemistry that can lead to ozone loss.

Susan Solomon in her laboratory at the NOAA Aeronomy Laboratory in Boulder, Colorado

Molina, Rowland and Crutzen were the first to identify the key catalytic processes. The Prize was a tremendous boost to atmospheric chemistry and to atmospheric science in general.

H.T. — Have you ever felt that, as a woman scientist, you have been discriminated against?

S.S. — I have encountered a handful of men who send out the message that they will never respect or listen to women. On the other hand, dozens of supportive male colleagues have taken pains to send the opposite message that they actually enjoy working with women scientists. Often, women are so hurt by the first kind of experience that they never recover from it and it scars their attitude. I never let those episodes bother me. You also have to keep a sense of humour. When I was in Christchurch, New Zealand, with a group of 16 about ready to go to the Antarctic, a reporter asked me how it felt being a woman working with all those men. That is a terrible question. You can either sound like a misogynist or a a nymphomaniac or a combination of both. So I gave him an honest answer. I looked around and said “Wow! They are all men aren’t they!” as if I had not noticed before, and everybody laughed.  I had a female student who was bright but, I felt, too quick to compromise.  That is perhaps a natural feminine characteristic, but it can be misinterpreted by male colleagues. I told her that it was fine to compromise, but that she must also listen to her inner voice and when she thinks she is right, to say so. That is something most women find difficult and one of the reasons that many women have a hard time in science generally.

H.T. — Did you have any supervisors or colleagues who guided you in your early scientific work. What was the impact they had on your career?

S.S. — Mentoring can play an important role,especially in science. To emerge as a lead player in science, you have to be able to participate, you have to be invited to the table, you have to begin to move in the circles of the people who are the world leaders—and mentors can bring you to that table to start you off. It is also important because there are many different styles. A mentor can help develop a personal style. A person who has had a major influence in my life in that way is Don Albritton, Director of our laboratory. A number of other senior people at the Aeronomy Laboratory have had a big influence on my thinking, such as Fred Fehsenfeld and George Reid. I feel the same way about David Gutman and Ray Roble. Paul Crutzen taught me how to think about science and atmospheric chemistry. He had decided to leave for Germany and was preparing for his departure, yet he freed himself from his obligations and spent an hour a day with me, which is phenomenal for a graduate student. He has fantastic chemical intuition.

H.T. — Do you enjoy being a public figure and a very high profile scientist?

S.S. — Much of it has been fun. It is satisfying to make a useful contribution to educating the public. One example is my work with the Smithsonian Institution on the "Science in American Life" exhibit in Washington DC. It includes a computer-driven display where children can learn about different scientists and I am actually one of them. The children find out about me and hear me speaking. I do public radio interviews. I get frequent requests for interviews but I am careful about which ones I accept.

H.T. — What about the other aspects of the problem? We make the best researchers sit on committees; we do not let them do research.

S.S. — That is a big problem and one I came to grips with early in my career, in the late 1980s. I soon realized that most of what committees do is not very interesting, nor even very useful. I think we have too many advisory committees these days. It is important to learn how to pick the ones that really matter. Scientists must learn to keep their calendars under control and avoid the temptation to squeeze in as much as they can. I see the value of being a spokesperson for the field and a role model; I also see the value of communicating with the public, but I enjoy doing my own science and I reserve time for that.

H.T. — Tell us about your one-year appointment as Acting Director for the Atmospheric Chemistry Division.

S.S. — It was an honour to go back to NCAR 20 years later in the position that Paul Crutzen had had when I was a student. NCAR is a great place and I had the opportunity to work with some fantastic people. But I knew that management was really not a role that I wanted for more than a year. I was 41 years old and I was receiving inquiries from people about management-level jobs because it is perceived as the “ladder”. Management fosters the careers of people in science and certainly people who are good at it make a great contribution. I accepted a one-year stint as an opportunity to experience it first-hand. But it is not my personal ladder. I find the scientific ladder much more rewarding. I am not happy if I am not spending a lot of my time doing science. I love programming. I can spend all day finding a bug in a computer program. Maybe some day I’ll change my mind. That will depend on how long I feel able to do work that is on the cutting edge. As scientists we are fortunate, we can evolve and diversify: we can be researchers, professors, mentors, senior statesmen, committee members. As you develop insight and experience, we can contribute on all these different levels. I do not have a set idea about what the long-term future is going to be but, right now, I am enjoying doing science.

H.T. — What do you see as kind of true benefit of science and why it should be supported by our society?

S.S. — When it comes to atmospheric chemistry, I think we have an important mission. I think of myself as a privileged person. At NOAA, we have a fair amount of flexibility in choosing the problems that we work on and we try hard to serve society. Atmospheric chemistry is my only interest. I have the satisfaction of doing things that are relevant. Having a goal and the idea of public service are absolutely consistent with what I do as a scientist. This planet supports many people and will have many more who will be putting all kinds of chemicals and substances in the atmosphere. The 21st century will have enormous opportunities for people in atmospheric chemistry. I have been to the Antarctic, I have measured the ozone depletion in mid-latitudes, a significant fraction if not all of which is related to CFCs. I think this is a clear and justified scientific statement. Now, what is going to be done about it? That part depends on your world view.

H.T. — What was your personal reaction to the Antarctic?

S.S. — The trip to the Antarctic is long and uncomfortable: nine hours from New Zealand in a noisy military airplane without proper seats. When you get there, it is –40°C or even colder. I hardly noticed that, however, because it was the most exciting, challenging, fantastic experience of my life. I have never seen such unspoiled, natural beauty. The intense purple and blue of twilight are incredible. The polar stratospheric clouds—those very clouds that facilitate the depletion of ozone—are wonderful. They resemble tiny suspended rainbows or pieces of rainbows. The challenge of working there was tremendous. I have driven in huge snowstorms, where I could barely see the flags marking the road. It was scary, but it was also fantastic and there was a tremendous sense of history. This is a place that has been explored only in the past century. People died in 1911 trying to get to the South Pole on foot, without radios. It is remarkable how far we have come. Nevertheless, on my last trip in 1996, my older back hurt with those uncomfortable plane seats!

H.T. — Tell us a little of your work on volcanoes.

S.S. — Together with my colleague, Dave Hofmann, I wrote a paper in 1989 in which I said that in the case of a large volcanic eruption, liquid sulphatic aerosols might cause major ozone loss at mid-latitudes via a process analogous to that in the Antarctic. That prediction was confirmed in the aftermath of the Mt Pinatubo eruption, when we saw major ozone loss over mid-latitudes. I’ve continued to study that chemistry and see how well we can simulate it quantitatively.

H.T. — Why does ozone loss occur in the lower stratosphere in middle latitudes?

S.S. — It is hard to explain with conventional chemistry why the ozone depletion goes all the way down to the tropopause for the same reasons that the Antarctic ozone was hard to explain. Lower down, there are fewer reactive species and more chlorine should be retained in the reservoirs. I believe that Cirrus clouds near the tropopause can be liquid as well as solid at times and sporadically cold and wet. Their surfaces catalyse a lot of the same chemistry as do Antarctic PSCs, and I think that is why ozone loss occurs down to the tropopause at mid-latitudes. I have started applying what we’ve learned about visible spectroscopy in the stratosphere to clouds and solar radiation. We can detect chlorine dioxide, for example, which absorbs only a few tenths of 1 per cent of the light in the stratosphere. It makes sense to apply these methods to investigate how light is absorbed in clouds, and by water molecules. That is what I enjoy about science, you work on something, building up and outwards, and always find connections to what you have done before.

H.T. — What areas of research have you been pursuing recently?

S.S. — In the last five years, I have continued with work on modelling stratospheric ozone, in particular, trying to pull together our measurements from the Arctic and Antarctic on chlorine dioxide. We have an interesting paper showing that those data support the importance, not only of heterogeneous chemistry on PSCs, but also on liquid aerosols. In the last two years, I have become intrigued by using the same kind of techniques that we have been using for 20 years to measure chlorine dioxide and ozone and nitrogen dioxide molecules that absorb visible light very weakly not to stratospheric chemistry but rather to clouds and climate. We are making measurements of elements like water vapour to see how much absorption of visible light there is in clouds. We are also measuring the absorption of sunlight by molecules such as O4 and oxygen. What is nice about it is that these molecules do not change when a cloud comes and goes. The absorption of oxygen increases, not because the oxygen itself is increased, but because the particles in the clouds are acting like tiny mirrors. There is a tremendous amount of amplification of the optical path in the presence of clouds. With those measurements I am trying to verify whether the way that meteorological models parameterize the absorption of radiation by clouds is correct. We have also applied the same kinds of techniques to the clear sky as well, to find out if there is significant absorption from the water vapour dimer. It is a sensitive technique that was developed for chemistry and which we are now applying to meteorology.

H.T. — How are you doing this work?

S.S. — In the laboratory, we have tried to make measurements looking directly at the rising Sun. This is because, when the Sun is right on the horizon, the path of the photons through the atmosphere is incredibly long. This enabled us to look for the water vapour dimer. However, most of our work has been done from aeroplanes. In May 2000, we flew on the NOAA P3 aeroplane with instruments looking both up and down and I am really excited about that database. We were able to see in some of the clouds we went through that the optical path was enhanced by more than a factor of 10, compared to clear sky, so the photons were passing through the equivalent of 10 atmospheres of absorption. The only way to understanding was to fly around, through, over and under clouds like that. I am doing this with a small team of four colleagues.

H.T.— Where do we stand as regards research in the ozone field, concerning generation, regeneration, disappearance or changes in the ozone? Is there still much more to be done?

S.S. — As Molina and Roland said, the important aspect of CFCs is their lifetime. It will take 40-50 years before we see the ozone regenerate, the ozone hole decline and mid-latitude ozone return to the levels of the 1970s. Interesting work has been done recently on whether the changes in carbon-dioxide and other greenhouse gases which cool the stratosphere might in fact slow down the reversal of the ozone hole. If the stratosphere gets colder, less chlorine may be needed to produce the ozone hole. By the same token, ozone depletion might occur in the Arctic, even if the chlorine drops. If the Arctic becomes much colder, the ozone depletion there might worsen. If the control measures needed for ozone recovery are now in place, it is a question of waiting to see the changes happen. People are impatient and ask whether the ozone hole is getting bigger or smaller this year. That is the wrong question to ask. If it is bigger or smaller, it is only because it happened to be a cold year or a warm year in the Antarctic. The year-to-year changes do not reflect a systematic recovery or a worsening. They have to be considered on a long time-scale.

H.T. — Does the variation in the atmospheric circulation affect the variation of the ozone?

S.S. — The ozone hole in 1988 was much less pronounced than in 1987 or 1989, because 1988 was a relatively warm year with an early stratospheric final warming in the Antarctic. Chemistry sets the stage for ozone destruction but meteorology modulates it, changing its nature from one year to another. It is remarkable how small temperature changes have a substantial effect on ozone. In the last few years, the Arctic has had a number of really quite cold stratospheric springs where the final warming was late and the ozone depletion was pronounced. In other years that is not the case; it depends on the meteorology. Of course, there would be no ozone hole at all if were not for the CFCs put there by human beings. Neither do volcanoes cause mid-latitude ozone change.

In the Antarctic

Even in the cold years, i.e. the 1960s, we did not see an ozone hole in the Antarctic; we only began to see it in the 1980s and 1990s as the CFCs climbed to higher and higher values. Cold temperatures do not cause the ozone hole, they modulate it; the volcanoes do not cause mid-latitude ozone depletion, they modulate it. Natural factors affect the damage that chlorine might make to ozone, but they do not cause the chlorine to be there: we are the ones who caused the chlorine to be there in the first place.

H.T. — What sort of contact have you had with WMO?

S.S. — The most important contact I have had has been through my work since 1985 on the WMO/UNEP international ozone assessments. Together with Sherry Rowland, I co-chaired the first formulation of the common questions about ozone. We put together one-page answers to the kinds of questions that the public asks, i.e. how do the CFCs get to the stratosphere when they are heavier than the air?; how do you know that chlorine is from CFCs?, etc. WMO has been an effective way of getting information to the public. The detailed information in the assessments has been absolutely critical, not only in bringing the international community together to develop a consensus on the state of the ozone layer, but also in helping us to formulate our views. I am also a member of the Joint Scientific Committee for the World Climate Research Programme.

H.T. — In March 2000 you were awarded the highest US national award.

S.S. — Our work to understand ozone is useful to the public and serves society. The National Medal of Science is a wonderful honour and the recognition of the work I have been doing for 20 years. Twelve scientists from various fields, such as medicine and physics, received the award from the President of the United States of America. I thought the other 11 persons were among the finest people I have ever met. It made me realize how important scientific research is; particularly when your nation appreciates what you have done.

H.T. — Tell us about one important and unforgettable event of your professional life?

S.S. — One of many was my arrival in the Antarctic. When the door of the aeroplane opened and that incredibly cold air rushed in and hit my face, it felt like being on another planet. Being in the Antarctic for three months was one of the most unforgettable experiences of my life. I particularly remember the colours of the sky and the most stunning twilight I have ever seen. Seeing the aurora for the first time was truly incredible. Many memories of the Antarctic will be with me for ever.

H.T. — If a young person asked you for advice as to how to become a successful atmospheric chemist, what would you say?

S.S. — My best advice would be that to be successful in science, you have to be focused. You have to be able to think and absorb science exclusively and constantly. It is difficult for young people today, they have so many distractions. They should not try to juggle science with sport and other pursuits. You cannot have a diversity of lifestyle and be successful. Those who come back to the field later tend to be more focused; they know what they want and that is an advantage. If science does not motivate you enough to make you want to centre your life around it, then it is probably not for you.

H.T. — Would you like to go back to the Antarctic?

S.S. — I would go back to the Antarctic any time. When I first went, I started reading about Scott’s expedition. He arrived at the South Pole a month after Amundsen. Amundsen came back but Scott died on the return journey. I have written a little book about what happened to him, and meteorology was an essential factor in his death. In a normal year, Scott would have survived his march back from the Pole, but he was extremely unlucky and hit a period of about three weeks when the temperature was persistently 5-10°C colder than what we now know is normal. This difference is a huge one and affects a person’s energy, comfort and ability to move. Scott writes in his diary about the amount of friction that the cold snow generated on the skis when he and his team were trying to drag their supplies. It is relatively easy when it is warm: the motion of the ski across the surface of the snow creates a thin layer of liquid water which makes you glide. When it gets very cold, the liquid layer cannot form and the friction on the surface increases.

H.T. — Allow me to congratulate you on a career which has been not only outstandingly successful scientifically but also personally rewarding.

  • 1 Interviewed WMO Bulletin 47 (2) [back]



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