The world has already passed the point of no return for climate change, and civilisation as we know it is now unlikely to survive, according to James Lovelock, the scientist and green guru who conceived the idea of Gaia - the Earth which keeps itself fit for life.
In a profoundly pessimistic new assessment,Professor Lovelock suggests that efforts to counter global warming cannot succeed, and that, in effect, it is already too late.
The world and human society face disaster to a worse extent, and on a faster timescale, than almost anybody realises, he believes. He writes: " Before this century is over, billions of us will die, and the few breeding pairs of people that survive will be in the Arctic where the climate remains tolerable."
In making such a statement, far gloomier than any yet made by a scientist of comparable international standing, Professor Lovelock accepts he is going out on a limb. But as the man who conceived the first wholly new way of looking at life on Earth since Charles Darwin, he feels his own analysis of what is happening leaves him no choice. He believes that it is the self-regulating mechanism of Gaia itself - increasingly accepted by other scientists worldwide, although they prefer to term it the Earth System - which, perversely, will ensure that the warming cannot be mastered.
This is because the system contains myriad feedback mechanisms which in the past have acted in concert to keep the Earth much cooler than it otherwise would be. Now, however, they will come together to amplify the warming being caused by human activities such as transport and industry through huge emissions of greenhouse gases such as carbon dioxide (CO2 ).
It means that the harmful consequences of human beings damaging the living planet's ancient regulatory system will be non-linear - in other words, likely to accelerate uncontrollably.
He terms this phenomenon "The Revenge of Gaia" and examines it in detail in a new book with that title, to be published next month.
The uniqueness of the Lovelock viewpoint is that it is holistic, rather than reductionist. Although he is a committed supporter of current research into climate change, especially at Britain's Hadley Centre, he is not looking at individual facets of how the climate behaves, as other scientists inevitably are. Rather, he is looking at how the whole control system of the Earth behaves when put under stress.
Professor Lovelock, who conceived the idea of Gaia in the 1970s while examining the possibility of life on Mars for Nasa in the US, has been warning of the dangers of climate change since major concerns about it first began nearly 20 years ago.
He was one of a select group of scientists who gave an initial briefing on global warming to Margaret Thatcher's Cabinet at 10 Downing Street in April 1989.
His concerns have increased steadily since then, as evidence of a warming climate has mounted. For example, he shared the alarm of many scientists at the news last September that the ice covering the Arctic Ocean is now melting so fast that in 2005 it reached a historic low point.
Two years ago he sparked a major controversy with an article in The Independent calling on environmentalists to drop their long-standing opposition to nuclear power, which does not produce the greenhouses gases of conventional power stations.
Global warming was proceeding so fast that only a major expansion of nuclear power could bring it under control, he said. Most of the Green movement roundly rejected his call, and does so still.
Now his concerns have reached a peak - and have a new emphasis. Rather than calling for further ways of countering climate change, he is calling on governments in Britain and elsewhere to begin large-scale preparations for surviving what he now sees as inevitable - in his own phrase today, "a hell of a climate", likely to be in Europe up to 8C hotter than it is today.
In his book's concluding chapter, he writes: "What should a sensible European government be doing now? I think we have little option but to prepare for the worst, and assume that we have passed the threshold.We will do our best to survive, but sadly I cannot see the United States or the emerging economies of China and India cutting back in time, and they are the main source of [CO2] emissions. The worst will happen ..."
He goes on: "We have to keep in mind the awesome pace of change and realise how little time is left to act, and then each community and nation must find the best use of the resources they have to sustain civilisation for as long as they can." He believes that the world's governments should plan to secure energy and food supplies in the global hothouse, and defences against the expected rise in sea levels. The scientist's vision of what human society may ultimately be reduced to through climate change is "a broken rabble led by brutal warlords."
Professor Lovelock draws attention to one aspect of the warming threat in particular, which is that the expected temperature rise is currently being held back artificially by a global aerosol - a layer of dust in the atmosphere right around the planet's northern hemisphere - which is the product of the world's industry.
This shields us from some of the sun's radiation in a phenomenon which is known as "global dimming" and is thought to be holding the global temperature down by several degrees. But with a severe industrial downturn, the aerosol could fall out of the atmosphere in a very short time, and the global temperature could take a sudden enormous leap upwards.
One of the most striking ideas in his book is that of "a guidebook for global warming survivors" aimed at the humans who would still be struggling to exist after a total societal collapse.
Written, not in electronic form, but "on durable paper with long-lasting print", it would contain the basic accumulated scientific knowledge of humanity, much of it utterly taken for granted by us now, but originally won only after a hard struggle - such as our place in the solar system, or the fact that bacteria and viruses cause infectious diseases.
Global warming, caused principally by the large-scale emissions of industrial gases such as carbon dioxide (CO2), is almost certainly the greatest threat that mankind has ever faced, because it puts a question mark over the very habitability of the Earth.
Over the coming decades soaring temperatures will mean agriculture may become unviable over huge areas of the world where people are already poor and hungry; water supplies for millions or even billions may fail. Rising sea levels will destroy substantial coastal areas in low-lying countries such as Bangladesh, at the very moment when their populations are mushrooming. Numberless environmental refugees will overwhelm the capacity of any agency, or indeed any country, to cope, while modern urban infrastructure will face devastation from powerful extreme weather events, such as Hurricane Katrina which hit New Orleans last summer.
The international community accepts the reality of global warming, supported by the UN's Intergovernmental Panel on Climate Change. In its last report, in 2001, the IPCC said global average temperatures were likely to rise by up to 5.8C by 2100. In high latitudes, such as Britain, the rise is likely to be much higher, perhaps 8C. The warming seems to be proceeding faster than anticipated and in the IPCC's next report, 2007, the timescale may be shortened. Yet there still remains an assumption that climate change is controllable, if CO2 emissions can be curbed. Lovelock is warning: think again.