Paul Crutzen was born in 1933 in the Netherlands and started his career as an engineer building bridges in Amsterdam. He moved into atmospheric science, and went on to become a leading expert on the ozone layer and many other aspects of atmospheric chemistry. He has published more than 350 research papers and co-authored or edited eight books. He now divides his time between the Max Planck Institute for Chemistry in Mainz and the Scripps Institution of Oceanography at La Jolla, California.
You鈥檝e had a remarkably eclectic career. How did it all begin?
I was a late starter in science. Unfortunately, or maybe fortunately, I failed my final exams to go to university in Amsterdam because I had a bad fever from a reaction to a smallpox inoculation. So I went to engineering school instead and then for four years, after national service, I was a bridge-builder in Amsterdam. But I loved winter sports and while in Switzerland I met a Finnish woman and went to live with her in Scandinavia. One day I saw an advertisement for a computer programmer in the meteorology department of Stockholm University. I had no experience of programming, but I got the job.
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This was pioneering work at the time, wasn鈥檛 it?
I guess so. I was using the fastest computers in the world and running the first numerical weather-prediction models. We had to do our own machine coding then, and I鈥檇 spend nights with the engineers, listening to the computer to see if everything was working right.
Not bad for a 26-year-old bridge-builder without a university degree. But you set your sights higher. How long was it before you started thinking about the ozone layer?
The work on weather forecasting attracted many scientists from the US. In 1965, when I was 32, I was asked to help one of them build a computer model of the ozone layer. The models then were all very simple, with very few reactions. But I started reading the literature and I got this strange feeling something wasn鈥檛 right with the theory of ozone in the atmosphere. We needed additional chemicals and reactions to explain it.
I thought the answer involved nitrogen oxide, though I doubted for a long time that I should publish my ideas. I had no data. Nobody knew then if there was any nitrogen oxide up there. And even though by now I had completed a mathematics degree and was 37 years old, I still thought that proper scientists were Einstein-like figures, not people like me. But I eventually got my paper published in 1970.
And soon afterwards, everybody started getting concerned about damage to the stratosphere from the supersonic planes like Concorde that were being planned鈥�
Yes, and because of my paper, I was the first to realise that their nitrogen oxide emissions might destroy the ozone layer. It was a very nervous time for me because I had stumbled on a hot potato but I had no formal background in chemistry. I had to teach myself the subject. In the end someone else, Harold Johnston, published first because the editing of my paper was held up by a postal strike in Oxford, where I was working at the time.
That must have been a big disappointment鈥�
Well, actually I was really happy to have support for my ideas from such an eminent scientist. Luckily large fleets of supersonic aircraft were never built. But soon afterwards interest moved to the effect chlorine compounds might have on the stratosphere.
There was a famous paper on this by Mario Molina and Sherry Rowland. When I read it, I was at first disappointed, because I too had been thinking about chlorine chemistry. But I hadn鈥檛 known about CFCs as a source of chlorine. I reported their finding at a meeting in Stockholm in 1974. There was a journalist there and that was how the whole CFC story became common knowledge.
I did the first model of chlorine in the stratosphere and calculated that it could destroy up to 40 per cent of the ozone layer at certain altitudes. It was, I guess, quite an important contribution. In the end, in 1995, the three of us got the Nobel Prize in Chemistry for our work.
But weren鈥檛 you written out of the script for a long time, with Rowland and Molina getting all the publicity?
Well, maybe so. But a decade later in the mid-1980s I helped work out why the ozone hole had been forming over Antarctica. It turned out to involve nitrogen oxide after all, because the CFCs destroyed ozone on frozen drops of nitric acid and water. That helped pave the way for the Montreal Protocol banning CFCs.
And then you came up with the idea of a nuclear winter, didn鈥檛 you?
Yes, that was my suggestion originally. People were worrying about the possible effects of nuclear war on the ozone layer. In 1981, the magazine Ambio asked me to write an article about it. I refused at first. I thought, wrongly really, that the bombs would have killed us all ten times over, so why worry about the pollution afterwards? But the editor persisted.
Then one day it struck me that in a nuclear war you would get fires everywhere. With a colleague, John Birks, I worked out that there would be so much soot in the atmosphere that it would become dark in daytime and temperatures would crash. These are very simple thoughts really. But in science you have to think the right things. Anyhow, it became big news. There was a lot of military interest, and it convinced me we have to abolish all nuclear weapons.
But again, you didn鈥檛 get the credit. The American scientist Carl Sagan took up your idea and got the headlines鈥�
Yes, that鈥檚 partially true. But this was such an important subject that it didn鈥檛 matter. And Sagan was a tremendous communicator, so I didn鈥檛 mind. He always recognised our contribution. I never had any bad feelings about it. I had been interested in other fires, too, such as those from burning forests and crop waste in the tropics. I had a paper published in Nature in 1980 which showed that there was so much biomass burning around the world that it was a major contribution to global air pollution and produced a great many very reactive chemicals that needed studying.
I gradually became convinced that aerosols of smoke from these fires could be influencing the climate almost as much as greenhouse gases. But we needed observations. In the early 1990s, V. Ramanathan and I proposed that we should investigate the huge clouds of haze coming from India and spreading over the Indian Ocean. It finally happened in 1999, when we had more than 200 scientists, three ships, and several aircraft and ground-based stations all sampling the air. We found that, both directly and by changing the optical properties of clouds, these hazes were cutting solar heating by 10 per cent or more, more than cancelling out global warming in many places. In fact, globally it seems that biomass burning may be shielding the planet from perhaps three-quarters of global warming. The climate scientists have not yet taken this properly into account (New 杏吧原创, 7 June, p 7).
So we could say that pollution from poor countries is saving the planet from the effects of greenhouse gases from the rich countries?
Yes, for now. But the problem is that most aerosol emissions stay in the atmosphere for only a few days. Greenhouse gases stay for a century or longer. So as this century goes on, aerosols will protect us less and less from global warming.
Does it worry you how little we know about the atmosphere?
Of course. We know surprisingly little about the risks we are running when we interfere with planetary processes on the scale we are now doing. We should never forget how lucky we were with the ozone layer. If Joe Farman and his colleagues from the British Antarctic Survey hadn鈥檛 carried on measuring ozone there in the 1980s when their bosses wanted them to shut down the project, we might only have discovered an ozone hole when it formed outside the Antarctic, perhaps over heavily populated regions.
And looking back, we were extremely lucky that industrialists chose chlorine compounds, rather than the very similar bromine compounds, to put in spray cans and refrigerators early last century. If they had chosen bromine, we would have had a catastrophic ozone hole everywhere and in all seasons by the 1970s.
You have said that the mid-latitudes don鈥檛 matter. How so?
I was being deliberately provocative. But it is true that the big planetary changes happen in the tropics and in the very high latitudes. The tropics are where the high temperatures drive a lot of the chemistry and dynamics of the atmosphere. And the polar regions, apart from being where the ozone hole happened, are the homes of the big natural feedbacks that could accelerate climate change: things like melting ice and permafrost, and alterations to ocean currents. If the permafrost melts, it could release huge amounts of methane, which is a much stronger greenhouse gas than carbon dioxide. That would be very dangerous.
Your latest find is more of an invention: the new geological age of the 鈥淎nthropocene鈥�.
This happened at a meeting three years ago. Someone said something about the Holocene, the geological era covering the period since the end of the last ice age. I suddenly thought this was wrong. In the past 200 years, humans have become a major geological force on the planet. So I said, no, we are not in the Holocene any more: we are in the Anthropocene. I just made up the word on the spur of the moment. But it seems to have stuck.
If you were starting out now in atmospheric chemistry what would you tackle?
I had great luck. To get started, you needed to know just four reactions. Now there are hundreds. It鈥檚 beautiful, but if you come in as a newcomer now, it鈥檚 very complex. Maybe I鈥檇 have been too lazy to tackle it now.
You seem to be brave. If you have an interesting idea, you pursue it even if you have no data. And you refuse to hunt with the pack. Why?
Perhaps it was being an engineer. I didn鈥檛 just observe things, I made them. Or perhaps it was being a meteorologist. The weather is very chaotic. Trying to predict it made me realise that, even using computers, you have to work with intuition. And I have learnt to question everything. Whenever a scientific paper says, 鈥渋t is well known that鈥�, I know there is probably some good research that needs doing to check that.
Perhaps that attitude is in your genes somewhere?
I don鈥檛 know. My father was a waiter. My mother worked in a hospital kitchen. They were fine people, but not academic. I had an uncle who played chess and was very interested in astronomy. But he was a coal miner and never had a chance to pursue academic studies. As a child, I read a lot about exploration. It was wartime and we couldn鈥檛 go anywhere. I knew only flatlands, so I imagined mountains. I thought that the tops of cumulus clouds were mountains with snow on. But I hope I don鈥檛 make those kinds of mistakes any more.