The hole in the ozone over Antarctica surprised a world that thought it was polluting only its cities. Paul Crutzen asks what other shocks lie in store.

From an evolutionary point of view, Homo sapiens is a unique and highly successful species. Its brainpower has led to great technological, agricultural and medical advances. With plentiful natural resources available, its population has expanded tenfold to more than 6 billion over the past three centuries, accompanied by a doubling of life expectancy in many parts of the world. This has happened despite frequent mass killings by the only species capable of developing weapons for self-destruction.

The "anthropocene", the current and in many ways human-dominated geological epoch, can be said to have started with James Watt's steam engine in 1784. During this period, there has been a substantial increase in several climatically important "greenhouse gases" in the atmosphere. The presence of carbon dioxide (CO 2 ) has risen more than 30 per cent due to the burning of fossil fuels and deforestation, while the presence of methane (CH 4 ) has increased 100 per cent, in part because of the estimated 1.4 billion cattle around the world (about one per family).

During the 20th century, industrial output increased 40 times and energy use was more than ten times higher than during the entire preceding millennium. Moreover, about 40 per cent of the continental surface has been modified by humans, causing major species extinction. And yearly, some 160 million tonnes of sulphur dioxide (SO 2 ), a precursor of acid rain, enter the atmosphere from coal and oil burning, more than twice the sum of all natural emissions. Releases of nitric oxide (NO), a catalyst in ozone formation, from fossil fuel and biomass burning, exceed natural inputs. All this human activity has had major effects on the environment.

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Only 30 years ago it was generally believed that mankind could cause regional air pollution only in urban areas, such as the ozone smog of Los Angeles and London's infamous winter smog. This reached its peak in December 1952 with the death of about 4,000 people in one night of unusually stable meteorological conditions that resulted in sulphur and other pollutants from home coal-fires being trapped low in the atmosphere. These were forewarnings of more widespread problems.

Despite significant improvement in Los Angeles, photochemical ozone smog now affects many urban regions around the world. In addition, during the dry season, in rural regions of the tropics and subtropics of Africa, Asia and South America, biomass burning leads to high concentrations of ozone and smoke. Although countermeasures, such as centralised power generation, higher chimneys and shifts to natural gas prevented a repeat of the London smog of the 1940s and 1950s, the result of growing fossil-fuel combustion after the war, acid rain became a major cross-boundary problem in the 1960s in Europe and northeast America, causing forest damage and fish death in lakes. After some relaxing of the problem in these regions thanks to reductions in SO 2 emission, the same issue is now affecting many parts of Asia.

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The possibility of global air pollution was raised in the first half of the 1970s with predictions that large fleets of supersonic aircraft and releases of chlorofluorocarbons (CFCs) could cause significant depletions of stratospheric ozone, nature's shield against harmful solar ultraviolet radiation. Catalytic ozone destruction by photochemically produced fragments of CFCs turned out to be a particular cause for concern. Its severity was much underestimated until in 1985 British scientists reported rapid ozone loss during springtime over Antarctica. This discovery and the scientific proof that the "ozone hole" was caused by chlorine-catalysed ozone destruction led to international regulations to stop the production of CFCs and several other ozone-depleting compounds by 1996. Unfortunately, because of the longevity of CFCs, the "ozone hole" will recur each spring over Antarctica - and is now estimated to extend over almost 70 per cent of the region - until at least the middle of this century.

The hole came as a total surprise. Nobody had expected maximum ozone loss over Antarctica, the farthest away from where CFCs are released in the atmosphere. What other surprises await us in the complex chemical/physical/biological web of interactions in the environment, and where are its weak spots? Can they be predicted? These are very important, but difficult questions to answer.

Two contrasting global issues are "nuclear winter" and "global warming". The former could result from a nuclear war in which large amounts of combustibles are ignited in cities and industrial centres, producing dark smoke in large enough amounts to shut off sunlight and photosynthesis, and to cause below-freezing temperatures over a large part of the continents, leading to mass starvation. In contrast, global warming is caused by growing amounts of greenhouse gases in the atmosphere, such as CO 2 , CH 4 and others. It was recently estimated by the Intergovernmental Panel on Climate Change that surface temperatures on average would increase by 1.4-5.8C by the end of this century, making Earth substantially warmer than at any time during the existence of Homo sapiens.

It may not be well known that air pollution in the third world is particularly high because of extensive biomass burning and low combustion and emission control technologies. It has been estimated that, in India alone, indoor air pollution causes the death of thousands of women and young children every year. On a much larger scale, a pollution haze covers most of South and Southeast Asia and much of the northern Indian Ocean during the dry, winter monsoon season. This brownish, soot-containing haze reduces sunlight and evaporation at the Earth's surface by more than 10 per cent, which may reduce rainfall over an already water-stressed Indian subcontinent. Chinese conditions are even worse, and heavy pollution is also found in large parts of Africa and South America, mostly as a result of biomass burning. These effects are expected to grow in future.

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Much on Earth is certainly not how it should be. Here I have addressed only a handful of atmospheric issues. There are many questions to ask. Will mankind be able to get rid of nuclear weapons? Will it be possible to reduce climate warming, sea-level rise and changes in weather and precipitation? The task is huge: to stabilise atmospheric CO 2 levels requires a 60 per cent reduction in fossil fuel burning, which is unrealistic with so many people living in poverty. Clearly, the industrial countries must reduce their use of fossil fuels. It is hard to be optimistic. With great difficulty, and without the United States and a few other countries, the richer nations could take only a first modest step towards reducing their greenhouse gas emissions in Bonn this summer.

There is little doubt that Earth's future climate, and world peace, will be increasingly determined by future developments in the highly populated poor parts of the world. To alleviate their problems, truly global cooperation and knowledge transfer is needed. This presents the international scientific community in particular with a massive task: to offer, in an otherwise highly competitive world, opportunities for education, training and scientific collaboration in the exploration of our planet, hopefully leading to proper management of the environment in the anthropocene.

Paul Crutzen was awarded the Nobel prize for chemistry in 1995.

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