Paul J. Crutzen


Interview, June 2000

Interview with Paul Crutzen by Astrid Gräslund at the meeting of Nobel Laureates in Lindau, Germany, June 2000.

Paul Crutzen talks about family background, early education and interest in natural science; his work in the Institute of Meteorology in Stockholm (5:02); his discovery (6:55); the ozone layer (15:42); the Greenhouse Effect (19:10); ozone holes (23:43); and the consequences of a ‘Nuclear winter’ (27:09).

Interview transcript

Dr Crutzen, I would like to begin to ask you something about your family background, where you grew up and where you went to school and so on?

Paul J. Crutzen: I’m born in Amsterdam in 1933; well until 1958 I was basically all the time in Amsterdam. My family was a worker’s family, my father was a waiter, a restaurant waiter, my mother did some work in hospital. Life was not always easy and of course from 1940 to 1945 there was the World War and the Netherlands were occupied by the German Nazi troops. So we had that experience also. So even after the war, of course, things were a little slim. Because of course everything had to be built up again.

So it was not an easy time in Europe particularly.

Paul J. Crutzen: It was not an easy start.

But so how did your interest in science then begin? Was it at an early age at school already or did it come later?

Paul J. Crutzen: Firstly, I did quite well in school. It was easy for me to learn, especially natural sciences but also languages. I was early interested in languages because my father was very good in French and he wanted to show this off on me so, we had some competition in that way, you can almost say. And my mother was born in Germany so I could speak German pretty good.

So you were multi-lingual from the beginning?

… that book fascinated me so much and things like that pushed me sort of in the scientific area. …

Paul J. Crutzen: Yes, I was basically from home to bi-lingual, but then through school and through contacts with my father also French and then English and later of course Swedish when I moved to Sweden. But natural sciences I became interested in, I don’t know how, it’s just by reading books and the first books maybe were about explorations you know, the people, the Jules Verne books, the stories about the march to the North and to the South Pole and so on. And I remember at home we had a fantastic picture book about Yellowstone National Park, it was black and white of course, but that book fascinated me so much and things like that pushed me sort of in the scientific area.

Then I understand that you did not immediately start to work in science, you were educated as an engineer from the beginning and also worked as an engineer as I understand it?

Paul J. Crutzen: Well ,subject which I was also interested in was bridges, of course in Holland you have plenty of bridges. So, that I read a lot about. When I did my final exams for entrance into the university, the last year of our high school, I was very sick and had to do my exams with very high fever over a two-week period and so I had to do it that way because if I didn’t do the exams I would have to wait another year. So, there were no other opportunities.

A tough system?

Paul J. Crutzen: A tough system … so feverish I went to the written examinations and then part of the oral examinations and my grades which came out at the end were not top, they were reasonable, but not top and not good enough to get a stipend for university studies which I needed because my parents were certainly not able to pay too much for me. So I then decided well, the other love is bridges. And I then studied bridge building and building of waterways, locks etc. in a school which is a high technical school but not the Institute of Technology so in between. It was a three year course of which the second year was a practical year so you earned some money of which I could then survive also the third year so I wasn’t so much of a burden to my parents. So that’s how I entered here.

But then you also moved to Sweden after this period of studies I realise?

Paul J. Crutzen: I met a Finish girl on the mountain top in Switzerland. She was vacationing and I was sort of hiking in Switzerland and we started corresponding and I visited her and then in 1958 we married and then we decided to move half way between Finland and Holland and that is Sweden, and I learnt Swedish, which is a reasonably easy language to learn. It’s not very difficult, the grammar is relatively simple. So I worked in a house construction company for about 1,5 years and then I saw an advertisement by the University for Stockholm, Meteorology Department. They were looking for a programmer, computer programmer, and although I had no background in that area and … but I thought well must have something to do with mathematics which I liked very much.

… I could not afford doing the more experimental courses …

I applied and among quite many candidates, they picked me for some reason. That was my big luck, because I then started working at the Institute of Meteorology of the university as a programmer, but they allowed me to go to courses in mathematical statistics and then meteorology, very theoretical courses. Because I could not afford doing the more experimental courses which take time, you have to spend a considerable amount of hours in the lab and I couldn’t afford that. One of the reasons why I became a theoretician is because of practical reasons.

So this is when your interest in atmospheric chemistry rised in this environment of meteorology or did it come later?

Paul J. Crutzen: It came later and initially I studied as I said mathematics, mathematical statistics and then finally I decided, let me try out meteorology. I must say it was a little bit of a shock because mathematics is so clean and mathematical statistics … and meteorology is a very much also an intuitive science. You have to bring in, you apply of course the laws of dynamics and thermal dynamics, but then there’s so many … the system around us is so complex, you have to be very, very intuitive to pick the right things.

And at the Institute after a while, and it’s about 10 years or so, well a little less than 10 years, I became a programmer for a US scientist who was coming to get his PhD at the Institute of Meteorology. So I did the programming for him, the subject was always ozone, was one of the first models of the virtual distribution of ozone which we developed. And while I was doing that I started studying photo chemistry and I got suddenly fascinated and in spectroscopy and so on. That became then my … and one of the very important things I found out is that people just repeated themselves. It is obvious that, these sentences that this and this is true.


Paul J. Crutzen: Well, that was no so obvious to me, so I started doubting.

You came with fresh eyes?

Paul J. Crutzen: Yes, well this is often the case and I was allowed to come with fresh eyes. I couldn’t have picked a better environment to do my studies than the Institute of Meteorology and there was a lot of support for what I was doing. I got more and more freedom too, not to programme for others but to do …

To really think about some new questions.

Paul J. Crutzen: For myself, so it was very generous. Maybe it was not totally legal.

Very good obviously.

… but the reaction rates were wrong, and that I discovered and I said: That must be something else …

Paul J. Crutzen: So I then discovered that the reactions which were then supposed to explain the composition of the ozone layer, the vertical distributions, that were insufficient. That reaction rates we used, which gave about the right answer, but the reaction rates were wrong, and that I discovered and I said: That must be something else and I then came upon the idea that nitrogen oxide were controlling the ozone in the stratosphere.

And that in natural circumstances that was the first thought but then I also very soon discovered that big fleets of supersonic aircraft were going to be built in the United States, Boeing and the Concorde’s between France and England and then also in Russia or the Soviet Union there were big plans. Hundreds or up to thousands airplanes would be flying in the stratosphere emitting NOx and that would break down ozone. Independent of me also a United States scientist, Harold Johnston, Berkeley, made this discovery that the purely theoretical ideas I had about the role of NOx suddenly became a big societal issue, whether you should build those planes or not.

Also the research in this area exploded, before that we were maybe a handful of scientists worrying or not worrying but studying the ozone layer and suddenly it became a big public issue and lots of research money was going into this direction.

Yes of course. But this is an interesting question: Did the society immediately see the serious consequences of this just because you were a scientist’s projecting something or that was accepted?

Paul J. Crutzen: That was surprising. It was surprising because I mean the ideas were on the table and very soon of course no measures were immediately taken. The industry was hoping to build these planes. But it was clear that major studies had to be conducted before this expansion of the aircraft lead would take place. And we did several years of research, three or four years on this topic. But at the end suddenly there came another problem which already existed namely the release of chlorine into the stratosphere and by the chlorofluorocarbons gases.

So that was next.

Paul J. Crutzen: So we layered the basis for an even worse concern namely the spray gas.

And they just came then. So but at this time then you did move to the United States at some point here.

Paul J. Crutzen: In 1974, I firstly … I was then a “biträdande professor” it was called, I was just elected to become and that I already had decided to try it out in the United States for a year in Boulder, Colorado, National Centre for Atmospheric Research and then, well one year was not enough so I did a second year and finally we stayed there for a period of six years and I became director also for the research division, International Centre.

But then what was it after you left Stockholm and went to the States that you did more serious work on the ozone and NOx or was that already?

Paul J. Crutzen: Before.

It was before.

Paul J. Crutzen: Now of course I was involved also in the studies on the effects of chlorofluorocarbons and then my interest also turned not only to the ozone in the stratosphere but also close to ground, in the troposphere and discovered that a nitrogen oxides did the opposite that they do in the stratosphere. The stratosphere be above about 20 kilometres the nitrogen oxide catalytically destroy ozone. But in the troposphere they catalytically produce ozone.

I see.

Paul J. Crutzen: And these two thoughts which make basically much of my work.

Why is that so? Why do they grow up can you explain that in simple ways or?

… but one did not consider that these reactions basically take place everywhere …

Paul J. Crutzen: Well, it’s not so simple, you need a blackboard to write some equations down. If we only would consider the chemistry of the nitrogen oxides and indeed ozone would be depleted everywhere. However, low down we also have the emissions of hydrocarbons from automobiles but even more by force and it is the products from the oxidation of the hydrocarbons together with the catalytic action of NOx which then creates ozone and of course you need sunlight. That’s why the photo chemicals were mainly due in summer on the very stable conditions when hydro carbons can accumulate in the lower atmosphere and also nitrogen oxides, then you get these. But this was known already for say Los Angeles area but one did not consider that these reactions basically take place everywhere.

Oh yes, it’s just a question of amounts that you have more in the big cities of the hydro carbons.

Paul J. Crutzen: But the total production of ozone in the troposphere is much more determined by what is happening in the wide world and specifically over Los Angeles or other parts of the world, so that was another idea I brought in and well, they were nice ideas and I sort of survived on them.

Quite obviously very important ideas that also had quite a large impact on how society dealt with these problems because I realise nowadays, we may still have supersonic planes of course but at least we don’t have much spray cans anymore.

Paul J. Crutzen: And very few supersonic planes, less than 10 instead of thousands or so.

And then if you look now at the problems of the atmosphere would you say that at least these questions are under control or would you say that we are still developing, you could say dangerous conditions which are the result of things that we do wrong. I’m not talking now about the green house effect, we’ll come to that later, but what about the ozone? Even if the ozone hole and those variations that we see, are they more or less natural and under control or are they completely uncontrolled.

Paul J. Crutzen: No, they are not natural, the ozone holes would not be there without the input of the chlorofluorocarbons by human activities in the atmosphere but we recognise that as science is established and we have a very good picture of what is happening and because of that also political decisions are more easily made. If you stand there and say well it is maybe so but maybe not I mean a politician won’t do anything.

Now initially we were in that state also with regards to the ozone hole but that didn’t last very long, it was a question of three or four years when we had all the pictures, the hole pictures together, it was so obvious that the chlorofluorocarbons were the culprit and then over a number of years and the emissions were reduced and since 1996 the chlorofluorocarbons are no longer produced in the industrial world and in the near future also in the developing world which anyhow have produced very little chlorofluorocarbons, there would also be regulations.

And would that be enough to heal the hole do you think?

… the ideas which have been spread around so far on mending the ozone layer have been totally unpractical …

Paul J. Crutzen: We cannot do more. There have been, it will take 30 even up to a 100 years before the ozone hole will have disappeared. But that’s … nature must have its way now because the chlorofluorocarbons gases are so diluted in the atmosphere that you cannot just take them out and destroy them. It’s too late. And all the ideas which have been spread around so far on mending the ozone layer have been totally unpractical. And mostly many of those are also lounged by former military laboratories who wanted somebody to save the world. But nothing practical.

There is no such way.

Paul J. Crutzen: No, no and in fact in principal there are ways but you don’t create even those conditions.

Yes so now we would.

Paul J. Crutzen: The energy needed to do that is so overwhelming that you create an in house carbon dioxide problem.

Another a worse problem perhaps. So then maybe we should spend a few minutes on the greenhouse effect and your opinions about the carbon dioxide problem and what’s your opinion of our future there?

Paul J. Crutzen: We have still … maybe you can repeat the question … We have still one, you asked me at the end, have you forgotten something and that of course is one thing that we should also talk about and that is nuclear wind.

Yes of course.

Paul J. Crutzen: Yes, okay, good. But the question is about the effect of the emissions of greenhouse gases in the atmosphere in climate and in my opinion there is very little doubt that there is an effect that much of the temperature increase we are seeing is coming from that. I mean it’s … you cannot say that this issue is as clear as in the chlorofluorocarbon issue but it is so more logical that we are heating up the atmosphere and the earth surface, they’re not. It depends … the warm temperature on the whole on earth, above freezing, 15 degrees above freezing, is simply due to the presence of greenhouse gases in the atmosphere. They serve as a blanket over us keeping the warm radiation coming from the earth trapped, in fact the energy is recycled five to six times, before it gets out into space. It’s a wonderful machine doing this.

And we are now adding greenhouse gases, why wouldn’t it become warmer, it’s so … I think the others should prove much more than those saying there is nothing there … than we, but still all the models we develop indicate the same. That doesn’t prove models of course can be deficient but altogether if you want to bet you would be rather stupid to say it’s not getting warmer it’s all likelihood the case. And we see it in the temperature records.

Yes for maybe 50 years ago the statistics were not so clear about that but I think they are now that it is definitely an effect.

… we are at beginning of the warming, it will accelerate …

Paul J. Crutzen: And we are at beginning of the warming, it will accelerate and that is inevitable if people in the developing world are going to use as much or approaching to use as much energy as we are using per capita in the industrial world. Countries like India per capita use only 10-20% of the energy which is used than the United States. And so they of course want to have a similar standard of living and inevitably that will lead to larger emissions so we should do every effort to reduce the emissions by saving energy, by developing alternative techniques which do not depend on burning of fossil fuels, whatever we can do. And maybe become a little more modest, we would be satisfied with somewhat less, but that’s very hard for people to do.

Yes perhaps not. It was probably easier to convince people not to use spray cans with freon in them than to convince them of not using cars or such. But of course one can never predict the future I suppose but I’m sure that one will realise as this process goes on that one has to be more and more strict about doing something in the developed countries, particularly we have to be going ahead, so to speak.

Paul J. Crutzen: Well, you said about predicting the future, we can of course, we don’t know everything and we never know whether there are effects which can come as a big surprises. For instance the ozone hole was far away from predicted by any scientist. It was a few measurements by colleagues of the British Antarctic survey over Antarctica which showed that ozone was going down. No model predicted that, it was thought a dead part of the world, ozone is in the most stable condition as you can imagine. Just exactly there ozone breaks down in the height region in which normally ozone is at the maximum and then suddenly a few weeks, two months later it is gone.

And how could that happen then? Does one know that now?

Paul J. Crutzen: What we had not considered is that ice particles which form in the stratosphere at low temperatures below minus 80 degrees that on these ice particles reactions take place, which lead to the conversion of relatively stable, we call it reservoir molecules, into very aggressive chlorine atoms and chlorine monoxide radicals which attack ozone very efficiently. And the formation of the ice particles is a, you can almost say natural, process, that is dependent on that’s what is happening over Antarctica because it’s simply cold in winter time, early spring. But the reactions on the ice particles involving chlorine, they are of course coming from human activities.

So even in the Antarctic there is the spread of this chlorine that comes from other parts of the world.

Paul J. Crutzen: The residence time in the atmosphere is of the order of 50-100 years. So there is plenty of time to spread from the middle altitudes in the Northern Hemisphere where they are mostly used into any corner of the world. You find them every day, you find them also in ocean water because they are not very soluble in water but there is some solubility and we can follow that. There are concentration in the oceans and that learns something about the flow in the ocean.

So that’s another aspect of course. It just so happens that while you have the coldest places on earth that’s where this ozone hole will form.

Paul J. Crutzen: And the other thing is that the destruction of ozone is dependent on the concentration of chlorine atoms and chlorine monoxide radicals to the power too.

Aha so it is a cooperative thing.

Paul J. Crutzen: And now we have about six times more chlorine in the stratosphere than under natural conditions, so the ozone destruction is say 36 times larger than naturally. Under natural conditions these reactions would not have … a very low probability. But because of our input of chlorine in the stratosphere these large amounts by small emissions every year but because of the long lifetime of the species adding up, adding up until we have now six times more chlorine in the stratosphere than under natural conditions. It’s amazing.

You mentioned one other thing you wanted to speak about when we talked about is different scientific topics. I had also another question about what is your major interest right now so to speak, but maybe you want to bring up this question to begin with?

… pollution is not only created by industry but also by mass burning …

Paul J. Crutzen: With nuclear wind, well, one of the things I have been very much involved in the past with studying the effects of by a mass burning in the tropics on atmospheric chemistry. It’s normally assumed that the tropics is a very clean part of our environment and people go on vacation to the most wonderful places and of course they are not thinking about pollution there. However, pollution is not only created by industry but also by mass burning and that is happening a lot in the tropics. We have deforestation activities, we have people burn just to cook or they burn to get rid of stubble or old wood or old plant material so they burn everywhere. In the Savannah regions of the world of the one about every year half the area is burnt to get rid of the high dry yellow grass which is not very fancied much by cattle for instance.

So this is a lot of pollution coming into the atmosphere in the tropics so that has been one scientific activity of me and related also to human activities but out of that and my interest in that came totally by chance one day thought that maybe the fires which would be raging following a nuclear war a major nuclear war in cities and oil refineries etc bringing large amounts of soot into the atmosphere. What that would do to the radiation balance of the atmosphere and what we found out and that was actually, that work was initiated by an invitation by Ambio, a Swedish journal issued by the Royal Swedish Academy of Sciences. They asked a number of scientists to think about what would be the environmental consequences of nuclear war. Initially I didn’t want to participate in this study. I thought well a nuclear war everybody’s killed five or six times over and I was in the belief that this would be true, but then finally I realised this is not true. You can kill one person or a number of persons six or maybe even more times over really, theoretically, but people are spread all around the world.

And you cannot wipe out human race just by nuclear war, you can make an awful world, but to wipe out humans altogether is a story which is not true. But then one day when I thought about it what is happening when all the soot gets into the atmosphere from heavy fires deposited higher up in the atmosphere and then we found out that would block sunlight from reaching the earth’s surface and we then would create almost something like an anti-greenhouse effect in other words that, because of that, the earth’s surface would cool very strongly and the higher layers up into the atmosphere where the soot is were warm.

So you get a temperature profile which is totally opposite to what is usual when it is warmer low down than higher up. So you turn it the thermo structure of the atmosphere upside down. And of course photo synthesis is reduced under these circumstances, then additional this became a big international study and additional facts were found for instance that temperatures at the earth’s surface may go down below freezing and that of course would be even during summertime this could happen. And that would of course then be disastrous for food production and so on. So suddenly this idea that mankind or a large part of it could be really at risk by nuclear war suddenly became a hypothetical reality.

By you could say the after the fact consequences on the environment.

Paul J. Crutzen: And that is dumped and goes under the name nuclear winter.

So that is a rather terrifying prospect or aspect or even worse than just the reality.

Paul J. Crutzen: It’s really no fun to think about the consequences, but when I look back it’s probably almost the most important work I have done. And it’s also a warning sign and not just for the past and present but also the future because nuclear bombs still are not abolished and there may be proliferation and maybe future use of.

So then I think I don’t have any more questions on my note book here, but maybe you just want to say something more, you could say, less pessimistic about our future disregarding now the nuclear war which we all of course hope will never happen.

Paul J. Crutzen: Of course we never know what the future will bring. There will be large … they are large problems to solve but we must hope that our coming generations will have learned from the past and will have learned from these horrifying predictions and things like this will not happen but I think for that reason science is needed to understand and to warn for the wrong side effects of technological development. Military developments are part of technological developments maybe implying the potentially negative side.

Okay thank you very much.

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