But James Lovelock, the celebrated environmental scientist, has a unique perspective on the fate of the Earth. Thirty years ago he conceived the idea that the planet was special in a way no one had ever considered before: that it regulated itself, chemically and atmospherically, to keep itself fit for life, as if it were a great super-organism; as if, in fact, it were alive.

The complex mechanism he put forward for this might have remained in the pages of arcane geophysical journals had he continued to refer to it as "the biocybernetic universal system tendency".

But his neighbour in the village of Bowerchalke, Wiltshire, the Nobel Prize-winning novelist William Golding (who wrote Lord of The Flies), suggested he christen it after the Greek goddess of the Earth; and Gaia was born.

Gaia has made Professor Lovelock world famous, but at first his fame was in an entirely unexpected quarter. Research scientists, who were his original target audience, virtually ignored his theory.

To his surprise, it was the burgeoning New Age and environmental movements who took it up - the generation who had just seen the first pictures of the Earth taken by the Apollo astronauts, the shimmering pastel-blue sphere hanging in infinite black space, fragile and vulnerable, but our only home. They seized on his metaphor of a reinvented Mother Earth, who needed to be revered and respected - or else.

It has been only gradually that the scientific establishment has become convinced of the essential truth of the theory, that the Earth possesses a planetary control system, founded on the interaction of living organisms with their environment, which has operated for billions of years to allow life to exist, by regulating the temperature, the chemical composition of the atmosphere, even the salinity of the seas.

But accepted it is, and now (under the term Earth System Science) it has been subsumed into the scientific mainstream; two years ago, for example, Nature, the world's premier scientific journal, gave Professor Lovelock two pages to sum up recent developments in it.

Yet now too, by a savage irony, it is Gaia that lies behind his profound pessimism about how climate change will affect us all. For the planetary control system, he believes, which has always worked in our favour, will now work against us. It has been made up of a host of positive feedback mechanisms; now, as the temperature starts to rise abnormally because of human activity, these will turn harmful in their effect, and put the situation beyond our control.

To give just a single example out of very many: the ice of the Arctic Ocean is now melting so fast it is likely to be gone in a few decades at most. Concerns are already acute about, for example, what that will mean for polar bears, who need the ice to live and hunt.

But there is more. For when the ice has vanished, there will be a dark ocean that absorbs the sun's heat, instead of an icy surface that reflects 90 per cent of it back into space; and so the planet will get even hotter still.

Professor Lovelock visualises it all in the title of his new book, The Revenge of Gaia. Now 86, but looking and sounding 20 years younger, he is by nature an optimistic man with a ready grin, and it felt somewhat unreal to talk calmly to him in his Cornish mill house last week, with a coffee cup to hand and birds on the feeder outside the study window, about such a dark future. You had to pinch yourself.

He too saw the strangeness of it. "I'm usually a cheerful sod, so I'm not happy about writing doom books," he said. "But I don't see any easy way out."

His predictions are simply based on the inevitable nature of the Gaian system.

"If on Mars, which is a dead planet, you doubled the CO2, you could predict accurately what the temperature would rise to," he said.

"On the Earth, you can't do it, because the biota [the ensemble of life forms] reacts. As soon as you pump up the temperature, everything changes. And at the moment the system is amplifying change. "So our problem is that anything we do, like increasing the carbon dioxide, mucking about with the land, destroying forests, farming too much, things like that - they don't just produce a linear increase in temperature, they produce an amplified increase in temperature.

"And it's worse than that. Because as you approach one of the tipping points, the thresholds, the extent of amplification rapidly increases and tends towards infinity.

"The analogy I use is, it's as if we were in a pleasure boat above the Niagara Falls. You're all right as long as the engines are going, and you can get out of it. But if the engines fail, you're drawn towards the edge faster and faster, and there's no hope of getting back once you've gone over - then you're going down.

"And the uprise is just like that, the steep jump of temperature on Earth. It is exactly like the drop in the Falls."

Professor Lovelock's unique viewpoint is that he is just not looking at this or that aspect of the Earth's climate, as are other scientists; he is looking at the whole planet in terms of a different discipline, control theory.

"Most scientists are not trained in control theory. They follow Descartes, and they think that everything can be explained if you take it down to its atoms, and then build it up again.

"Control theory looks at it in a very different way. You look at whole systems and how do they work. Gaia is very much about control theory. And that's why I spot all these positive feedbacks."

I asked him how he would sum up the message of his new book. He said simply: "It's a wake-up call.''