As the 90-year-old father of Gaia prepares to blast off from Earth on the inaugural Virgin Galactic flight, he reflects on his own mortality, the future of our planet and wind farms – 'Monstrously silly!'.

Tolkien would have been happy here, or one of his hobbits. We are in low, comfortable seats in a quiet study in a house in a hollow in Devon. There is tea coming, and biscuits. But we are talking endgames; his and ours. It is one of the most terrifying conversations I have ever had.

James Lovelock, who turned 90 at the end of last month, knows his own endgame could be looming. He is booked on the inaugural flight of Virgin Galactic, a gift from Richard Branson: a flight into space from New Mexico, final tests for which are under way. His doctors have advised him that the risk is too high and he tells me in passing that he had a heart attack in 1971 – he'd been smoking for 34 years until that day – but he is determined to go, come what may. "To see the Earth, from above, before it vanishes."

He must surely, I ask, trying to be tactful while his wife, Sandy, is busy in the kitchen, have pondered on the high chance he won't come back; may not even survive the G-forces? "Hell, yes! But what a way to go! So much better than dying slowly of some hateful disease. And I get to see what I've always wanted to see."

What he's always wanted to see, since he formulated the theory back in the Sixties and Seventies, was Gaia: the Earth as an entity which could be described as "living". "Gaia theory", while once ridiculed, is now accepted by most scientists, so many of its predictions having objectively been seen to come to pass.

Gaia is the idea of a self-regulating Earth. Earth is not habitable because of "luck", but because life maintains its habitability. Living organisms, consciously or not, manipulate the oceans and atmosphere, even the (eventual) composition of rock, in order to maximise the conditions for the continuance of life.

Into this has come the growing realisation that we, humanity, have become the Earth's heaviest polluters. Our industrial society, indeed our very existence, is changing the CO2 balance around the planet to a degree now affecting its heat stability. Lovelock has been one of the prime thinkers behind the study of climate change and today he is, as it were, cheerfully gloomy. It's a mood that pervades his latest book: rational, widely read, witty, apocalyptic. His previous Gaia book was described as "the most important for decades"; his new one, The Vanishing Face of Gaia, makes its predecessor look like Teatime with Flopsy Bunny. His is the gloom that comes with hard-won knowledge.

"I think it's happening sooner than many of us realised," he says. "I would actually say that we'll start noticing things, in London for instance, within about 20 years." Both in interview and in his book, he says that scientists have been shocked at how wrong they were about the speed of change: observation and analysis – the closest he has to "gods", trusting them far more than computer models and academic theorising – show, for instance, that in 2007 the amount of floating Arctic ice that melted was three million square kilometres greater than expected, about 30 times the size of England.

The chances are now very high, he says, that the Earth, "in its but not in our interest", will be forced into a hot epoch: "a barren state in which few of us can survive". Note: not quite Armageddon; humankind can survive, he stresses, but only some of us. Those best suited, those who plan, and those lucky enough to live on what he calls the "lifeboats", the smaller temperate islands able to maintain a rain supply: Japan, New Zealand, Tasmania, Hawaii, perhaps. And Britain.

He stops short of saying, categorically, that we have passed the tipping point, but it is clear that this is his private belief. Yet he's far from given up, working hard still on schemes to slow global heating – he insists on this phrase, rather than the milder "warming" – such as ocean pipes to fertilise algae with cold water from the seabed and self-powered ships travelling the seas forever, seeding the clouds for rain. But, today, he is pragmatic. "We can't just turn the power off. If we stopped using power, if London stopped consuming power, in the world we've now built, it would be like Darfur within about two weeks. Our wish to continue business as usual will probably prevent us from saving ourselves."

Here, he says, the threat to London is the biggest one. "Research has shown that glaciers can go quite suddenly and quickly... we depend so much on London. Think of a Thames flood – tube system out, sewage everywhere, that's just the beginning. All of the land north of Cambridge, for instance, is already below sea level, some two metres below... I talk about the flooding here, but the rest of the world will have it rougher, because it'll have drought. As the earth gets hotter, people will want to move to these 'lifeboats' and that will bring so many problems of a different kind." He smiles benignly.

He hates being the bearer of this news, but he has the curse of knowledge. He knows exactly whereof he talks. Although best known to the layman for Gaia, Lovelock has been well-respected by other scientists for decades. A chemist by training, he was invited on as an "experimenter" in the infancy of Nasa, working at the jet propulsion laboratory in Houston. He recalls this time with delight: the early days of space travel technology. "They were often oddballs, real eccentrics then, people who had worked with steam power or clocks. Everything was exquisitely engineered, beautiful. And it worked: well, we saw that it worked. And the knowledge was real hands-on knowledge. I remember once seeing the screws they were using on the space vehicle and realising they were cadmium-plated, and knew that cadmium would evaporate in space." Seconds into the beyond, the craft would, literally, have a screw loose.

One of his early tasks was designing (and making) the tools to be used to gather dust from Mars. He became aware, here, of the huge import being attached to the search for life. "The biologists Nasa had looking for life were doing so in a poor way, in my opinion. This annoyed them a bit so they said, OK, clever clogs, what would you do? I told them to look for entropy reduction, not the specific life-form. My idea was to analyse the chemical composition of Mars's atmosphere. If there was life, it was bound to use the atmosphere to survive and that would have changed the atmosphere."

Had there ever been life, he reasoned, it would have broken down or altered the preponderance of CO2, as happens on this planet through. There was no need to land, simply to analyse the atmosphere. "Four years later, I was in an office with Carl Sagan when the latest data came in on atmosphere composition on Mars and Venus. I knew then that Mars was lifeless. Precious little but CO2."

The side-effect of this episode was the genesis of what became Gaia. "'Why was our atmosphere so rich, in contrast?' I kept asking. It dawned on me the atmosphere's being made, being maintained, by the life it itself maintains. Life, the possibilities for life on Earth, must be self-regulating. Not just the atmosphere but the temperature, which has stayed constant for at least millions of years, even though the Sun's warmed up."

His friend and then-neighbour William Golding, who knew a thing or two about imagination and apocalypse, suggested the name Gaia for the theory and it caught on quickly, though not always in the circles Lovelock would have preferred. "The new agers took it up. The crystals and homeopathy crowd. Oh, I don't really mind. People can believe what they want, no problem, unless it hurts someone else." More important, however, it was roundly dismissed as semi-fictionalised hokum by many scientists, including Richard Dawkins; its very name, while popularising the theory with laymen, irritated empirical thinkers.

"Biologists in particular hated it. People think that I'm anti-biologist, but… well, I've had my run-ins, but I'm not. But they do have a very dogmatic definition of life – it must contain DNA, it must reproduce. I think it should encompass large entities like ecosystems. The atmosphere, the oceans, the first few miles of surface rock. As alive as the cells in your toes are alive, and part of you, and you are alive."

In just over three decades, the theory is now close to scientific orthodoxy; all around us, the planet is seen to adapt to the changes we have wrought. But Lovelock's still thinking about it, refining it.

"Since we're a part of it, the planet, it's starting to strike me, it now knows. Knows us. We are not too far perhaps from becoming a proto-intelligent planet, a planet taking early steps in thinking for itself. This is much more recent thinking: if I hadn't been battling with biologists I would have understood it a lot quicker. But you can't speed up science."

Lovelock's ceaseless brain, its thrilling mix of imagination and pragmatism, has made him a hero to the ever-growing green movement. Apart from anything else, his invention of the electron capture detector (EDC) in the late Sixties allowed the world to test for and confirm the growing presence of CFCs in the atmosphere; the subsequent ban on CFCs was just in time to repair the hole in the ozone layer. But Lovelock is first and foremost a scientist: "I love being a scientist. I don't like it when the Gaia movement is treated like a religion, as if scientists are seen as part of the problem rather than the solution.

"Some of the more recent green hysteria is plain wrong. I know what there is to be worried about and what not to. Flying is not a major problem, not compared with all the CO2 being given out all the time by us and our pets. Flying's only got this reputation because often greening mixes with the bad side of the left, which is to do with envy. A lot of people have closed their minds to arguments against wind farms. But they are monstrously silly! A 500-ton concrete base support and 4,000 of them needed to equal the output of one coal-burning station – how is that helping? There was a report in Der Spiegel saying that in Germany, where they've got 17,500 of the things, the amount of CO2 now being produced in the country is greater than ever."

So what other solutions are there? "Tidal's a good idea, yes. But it would take time. We don't have time. Nuclear is the answer. Far, far less dangerous than any propagandist has ever pretended. Look, even now, at the wildlife all around Chernobyl! Because man and his pets have not been near for years!"

Saving spaceships, disproving life on Mars, finding the ozone hole and formulating perhaps the most important scientific theory of these, the end of days: it's not a bad turnout for the son of an apprentice poacher from Wantage. I remind myself, in the long taxi ride back to Exeter, re-reading the end of Lovelock's astonishing final warning of a book, that there is, at least, a little hope. We will, he thinks, survive. But not all of us. Perhaps a very, very few. I have rarely left an interview so elated by humankind's potential – for wisdom, logic, invention, charm, genius. Nor so aware of the world around me. Nor so devastated by the future.

"Oh, I do want to come back," he had laughed, about his space trip. "The secret of enjoying old age is always having something to look forward to." I hope he comes back, too, because I want him to be first on the lifeboat. And I hope he enjoys the view: of a world, we now know, which may need to get rid of us, or change us, but which will turn and twist and fight to keep breathing, and to keep life upon it, until (this time in the words of a farmer's son from Ayrshire), finally, a' the seas gang dry.

(By Euan Ferguson, The Observer, 09/08/2009)