We may be trashing our environment now, but the more crowded the world becomes, the less polluted it might be. Fred Pearce explains how.

It is an obvious proposition. More people are bad for the planet. Raise population density and you cause more pollution, wreck ecosystems and generally foul the nest. And that, I think, was the presumption behind the decision of editors at the American Association for the Advancement of Science to ask population writer Paul Harrison and me to provide the text for an atlas of population and the environment. But what we found in the course of preparing the book was a more complex story.

Here is a conundrum. Some of the most resolutely trashed places on earth are also some of the emptiest. I once spent a fortnight travelling through the tundra wastes of central Siberia. Nobody lives out there amid the bogs and mighty rivers. But that meant there were no limits on the amount of mess the Russian oil industry felt able to generate. As a result, the entire natural landscape for hundreds of kilometres around the oil city of Noyabr'sk was carved into tiny fragments by roads and pipelines, survey and power lines, drilling pads and embankments. What remained was smeared with oil, littered with vast amounts of discarded engineering trash and lit by hundreds of gas flares.

Or take the Brazilian Amazon basin. Population density is among the lowest anywhere. The distances are immense. But when there is unlimited spare land, there is no need to protect, husband or nourish it. Invading farmers burn out a stretch of forest, farm or graze it for a few years, and then move on. It is a formula being repeated across the tropics: in Indonesia's Borneo and Sumatra, and, increasingly, in the rainforests of central Africa.

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And even where there are substantial local populations, environmental damage is often not their fault. Go to the Niger delta in Nigeria, as I did this year. The oil spills that pollute the soils and the flares that light the sky have nothing to do with the local population - indeed, the local population has notoriously gained not a halfpenny's worth from the huge revenues generated by 40 years of oil exploitation in the delta. It is as if they were not there.

Of course, there are explanations for this conundrum. The damage is in essence displaced. The Siberian and Nigerian oil is being pumped for people thousands of kilometres away. The cattle are grazing the Amazon to feed the burger-munchers of North America and Europe.

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But, even so, the absence of people creates a climate in which maximum environmental degradation can take place. Why bother to clean up when there are few outsiders to see, let alone complain? And when the wilderness always appears endless, trash this bit and there will always be some more. This is the wild west, the outback, the jungle, the wastelands. And we respond with a "cowboy economy" and the "law of the jungle".

But this, depressing though it appears, gives us hope. The lesson is that as the wilderness ends, as enclosure replaces common land, as the land becomes owned and valued and scarce, so we take more care of it. The same oil company that will ride roughshod through the rainforests of West Africa will tiptoe with immense care through the pastures of Dorset. And what applies at local level should also apply at global level. As the world becomes more crowded we take more care of it.

Look at Europe and North America today. Our city air may not be perfectly clean, but it is much cleaner than half a century ago, when London was, along with much of urban Britain, still the regular victim of "pea-souper" smogs from coal burning. Similarly, fish have returned to most of our rivers. Birds and river life such as otters that were once killed by DDT and other virulent pesticides are now returning. Even acid rain has lessened. There are many more trees in our countryside than for many decades - and in some cases, many centuries.

These are certainly much more managed landscapes than their predecessors. We are replacing wilderness with garden. Indeed, most of the plant biological diversity in Britain today is to be found in our suburban gardens. But in many respects the story of the industrialised world over the past half century is an environmental success story.

There could be a universal, and optimistic, law here. As economies develop and populations grow, we seem to go through a phase of trashing the environment and being profligate polluters, before coming out the other side with stable populations, much greater efficiency in the use of resources, greater concern for our living environment and, in some respects at least, a lessening impact on the natural environment.

This can even be seen applying to perhaps the most intractable environmental problem in the world today - the global warming caused by accumulating greenhouse gases in the atmosphere. In the early days of industrialisation, pollution from these gases soars - partly because of rising populations, partly from increased wealth that raises per capita emissions and partly also because of a rampant inefficiency in the use of energy.

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Then the tide turns. In most of the industrialised world today, population has stabilised. And energy is used much more efficiently than before - more dollars of gross national product are created for every tonne of carbon dioxide poured into the air.

Similar things are starting to happen in the developing world. In perhaps the most dramatic single environmental statistic to emerge this year, United States government scientists revealed back in April that China's emissions of carbon dioxide fell by 17 per cent in the second half of the 1990s - at a time when its GDP grew by 36 per cent.

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Now I do not want to appear overly optimistic. China's improvement comes from a position of having one of the most inefficient (as well as largest) energy industries in the world. And it arose largely through shutting numerous uneconomic coal mines and tackling the chronic smogs in its cities. Doubtless at some point China's emissions will start to rise again.

But it does provide hope and a counter to the feeling that there is something inevitable about a worsening environment. And with the rate of the world's population increase now slowing, we can look forward to a stable population well before the end of this century, with perhaps falling levels of pollution.

None of this is inevitable. To those who argue that the environmental crisis is some kind of a hoax, my reply is that a sense of environmental crisis is an essential part of its solution.

And the natural world is not duty bound to respond in a predictable or benign way. Unexpected disaster is an ever-present risk. We nearly wrecked the planet's ozone layer without even knowing we were doing it, for instance.

And we are doing much that is virtually irrecoverable. It is clear that continued thermal expansion of the ocean depths and unstoppable melting of ice sheets will ensure that sea levels carry on rising for centuries after we halt global warming. Meanwhile, our once diverse natural world is becoming a series of mongrel ecosystems as our penchant for trade and travel takes exotic species round the globe, giving us the perverse outcome of greater local biodiversity combined with smaller global biodiversity.

Paul and I may have begun work expecting a near-perfect fit between population and pollution. But we found things are not that simple. The complexity raises important questions, offers important hope, and makes for a more interesting atlas.

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Fred Pearce was a principal contributor to the AAAS Atlas of Population and Environment , published by California University Press, ?45.00 and ?19.95 (paperback).