The air on Chesil Beach is uncommonly still. Eighteen miles of open sea and no waves; the shingle bank is a sun-trap, and there’s no sound – not even a seabird. The temperate nature of James Lovelock’s Dorset home turf is, he’d be the first to point out, a result of pollution. Ancient pollution. Between ice ages the levels of carbon dioxide (“that truly malign waste”) rise considerably. If Gaia – as Lovelock likes to call our Earth – had returned that carbon dioxide to the atmosphere as gas, we would have been as hot and dead as Venus a long time ago. Instead, she deposited it as calcium carbonate, in the chalk and limestone cliffs that line the south coast. In doing so, the planet kept her temperature down.

Along the shale bank are two concrete Second World War pillboxes and the discarded silver foil of someone’s portable barbecue tray. James Lovelock might be less bothered about this small sign of human disdain than some environmentalists. Catching the light with a white ray, the barbecue looks like a mini prototype of one of Lowell Wood’s space mirrors – the giant wire meshes the American astrophysicist would like to see launched into space to deflect heat from the sun. Diverting just 1 per cent of sunlight, they say, could stabilise climate change. Or, why not spray some of this sea water straight up into the atmosphere to produce clouds, mimicking the cooling effect of volcanic eruptions?

This, as Lovelock has been saying for many years, is the scale we should be thinking on. But now at the age of 100, he’s come to accept that humans are unable, possibly just too dumb, to carry out such feats of geo-engineering without help from a robot population. His Gaia hypothesis, conceived in the Sixties and published in the Seventies as a series of highly readable books and papers, suggested that the Earth is a single, complex, self-regulating organism, sustaining conditions for life. The science world called it unscientific. One of Lovelock’s early detractors, the American biologist Ford Doolittle, added that it was dangerous to mislead a lay readership about “the extent to which the biosphere can be safely abused”.

Yet Lovelock became an icon of environmentalism: George Monbiot says he has contributed more to our understanding of the planet’s reaction to global warming than any other living person. Lovelock was wrong, he himself admits, not to worry too much about climate change. Because Gaia, like him, is getting old. As his body could not withstand a car-crash now, the Earth – destined to get hotter and hotter – might not withstand an asteroid, or a huge volcanic eruption.

His cottage lies a hundred yards from the beach and the door is open. Lovelock is wearing a cardigan and specs, and takes his seat amid stacked bookshelves, next to a painted rocking horse. When I last spoke to him, ten years ago, he was just about to be flown into space by Richard Branson as a 90th birthday present. Stephen Hawking – whom Lovelock held as a baby, while working with Hawking’s father at the National Institute of Medical Research (NIMR) in London – was also due to take one of the 2.5-hour flights on the Virgin Galactic Spacecraft. But the long-delayed expedition has yet to take place. “They bust the airplane, had a nasty accident,” he laughs. “Apparently I’m still on the list but it’s getting pretty dated now.”

Lovelock wasn’t worried for his health in orbit, but he’s recently recovered from a bout of pneumonia that left him incapacitated for the best part of two years. He knew about an antibiotic, azithromycin, which he says would have cured him in a jiffy, but the National Institute for Health and Care Excellence had put a ban on it for anyone who’d suffered from heart problems. “The barmy NHS provided me with opiate drugs instead,” he says. “It was all so unnecessary. I grew less and less able to do anything. It was civil servants going mad as usual.”

He blames the opiates for damaging his memory for dates. “It made a handicap I had before much worse. When I was a schoolboy I couldn’t do arithmetic. But when it came to algebra, I found it easy. When I came to calculus that was just a doddle. And quantum theory – forget it, that’s obvious!”

In fact, his memory gaps would be natural for someone a couple of decades younger. In 2010, Lovelock said: “I have a feeling that climate change may be an issue as severe as war. It may be necessary to put democracy on hold for a while.” It is strange to be in the presence of a human being a century old who is out of touch with news stories such as Extinction Rebellion, yet remembers exact conversations from a balcony in Hampstead in 1942. “I must be tough or you wouldn’t be here now [to visit],” he says. “I can still walk five miles if necessary.”

Lovelock’s chuckling demeanour has confused people over the years; it is so at odds with his draconian predictions – most of the world will be uninhabitable by 2040, 80 per cent of humans will perish by the year 2100, with a few breeding pairs relegated to the Arctic. He is an incurable optimist delivering the greatest conceivable doom. He knows this, and can do nothing about it, telling the Guardian in 2016, “One may say: ‘Well, of course, he’s so old he’s stopped having any feelings.’ Not true, I’ll say!”

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He reaches for a photograph album from his days at the NIMR. He was hired aged 22 on the advice of his Manchester professor, Alexander Todd, who won a Nobel Prize for his work on nucleic acids – “because they needed a youngster who was practical, not too academic, doing ad hoc problems that need to be solved yesterday. Todd said: ‘This one’s no good at academic chemistry, so let’s send him.’”

Lovelock did not have a straightforward education. His father was born on the Berkshire Downs in the village of Fawley, which gave its name to Jude Fawley of Thomas Hardy’s Jude the Obscure. Thomas Lovelock did time in jail for poaching rabbits (“he was the original hunter-gatherer”). Although he was illiterate until he attended technical college, he was a natural educator, taking his son to the Natural History Museum, where the seven-year-old was more interested in Newcomen’s atmospheric steam engine than the bodies of “those old lizard skeletons” (dinosaurs). The family lived in Brixton, south London, where Thomas set up an art shop. James performed poorly at school in Tulse Hill but borrowed extensively from Brixton Library and took extra evening classes in science at Birkbeck College. He read chemistry at Manchester University, graduating in 1941.

His Royal Society nomination from 1974 spans a bewildering array of research areas – respiratory infection, blood-clotting, cryogenics, artificial insemination, gas chromatography, planetary atmospheres, pollution and global warming. He explains his status as a cross-disciplinary scientist – tongue-in-cheekly – as a result of the building where he started out. At the NIMR headquarters in Hampstead, his lab lay one floor below that of Archer Martin, inventor of the gas chromatograph. At one point, Lovelock was engaged in early cryogenics, freezing and resuscitating hamsters. He suspected that Martin’s chromatograph might help him analyse the fatty acids in the hamsters’ blood. He went upstairs to borrow it – but the appliance was not sensitive enough, so he set about designing another one (“I was getting terribly bored with biology”). His invention eventually evolved into the Electron Capture Detector, which he used to detect CFCs in the Antarctic. His research fed into work on ozone depletion.

Lovelock is the link in the chain between the various Nobel prizes, yet he has never won one. He had the hunches, refined the equipment – but he does not call himself a scientist: in recent years he has taken to calling himself an engineer. His detector was later sent to Mars, on the Viking mission. During his work at Nasa, analysing the Martian atmosphere, he observed the ways in which planet life organised its environment – the start of Gaia theory.

Lovelock had a good war. At the NIMR, during the Blitz, he was regularly enrolled as a firewatcher, stationed on the roof with a leading scientist of some kind, looking out for firebombs. His companion, he noted, would loosen up with sheer relief every time a bomb missed the institute and fell somewhere beyond Hampstead Heath instead. One bomb came so close, he saw the rivets: “I’m afraid to say, I was fascinated.”

In a photograph from his album, he stands with curly hair and tortoiseshell glasses in ill-fitting army fatigues. “We’re in battle dress. Don’t forget, clothing was rationed. When you travelled around the country, MPs got into a tizzy because you had no papers, no nothing – were we spies?”

On the facing page, an explosion emanates from between the husks of two buildings: “Canning Town. It was used for street warfare training – there’s a flame-thrower coming down the street, and I’m holding the camera.” He also held the camera on an aircraft carrier in the Arctic. Monochrome sea lashes at the floating platform: “This is a sad picture,” he says. “Within seconds of me taking it, all four people on that plane were dead – the carrier shifted with the wave and the plane fell into the sea.”

He turns the page to a shot of black smoke billowing on open water. “Churchill had an idea that if you set fire to the sea, the Germans wouldn’t be able to invade,” he says, blankly. “I towed a rowing boat through those flames to demonstate that they didn’t even damage it – the wave pushed the flames aside. Utterly crazy. But he was a crazy man. Mad, mad, mad, mad man.”

He shows me a final photo: a landing craft firing a blizzard of rockets. “The weird thing about this is, the skipper of one of those things was William Golding…”

Lovelock befriended Golding, who won the Nobel Prize in Literature in 1983, when they found themselves neighbours in the village of Bowerchalke, Salisbury, in the 1960s, and met on a walk to the post office. Golding had studied physics at Oxford before the war but returned to take English literature. He gave Lovelock the name “Gaia” – the mother goddess of the Earth – for his theories of the biosphere. In his Nobel acceptance speech, Golding said:

Now we, if not in spirit, have been caught up to see our Earth, our mother, Gaia Mater, set like a jewel in space. We have no excuse now for supposing her riches inexhaustible nor the area we have to live on limitless because unbounded.

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Golding was speaking a couple of years before the international Gaia conferences of the 1980s. The science world argued over whether Lovelock’s ideas – shared and honed with a co-conspirator, the maverick American scientist Lynn Margulis – were useful or completely unscientific, quasi-religious, even pagan. The evolutionary biologist Stephen Jay Gould said that the hypothesis was just a metaphor – it didn’t show the mechanism of how self-regulating homeostasis was achieved. But “mechanism” is itself a metaphor, countered the ecologist David Abram – and science needs organising principles. Those veering towards philosophy seemed to like Lovelock’s theories – including the sociologist Bruno Latour, who sees it as the successor to Galileo’s vision of the solar system. But the biggest criticism from scientists, including Richard Dawkins, was how un-Darwinian it was. If Gaia was an organism, why couldn’t she reproduce? And how could natural selection on individual organisms also work on a planetary scale? “They had this daft idea that it wasn’t Darwinian,” says Lovelock. “It’s dependent on Darwin!”

In the 1990s, the evolutionary biologist WD Hamilton called the Gaia theory “Copernican” in its scope – it would take another Newton, he said, to explain how the Earth’s self-regulation takes place through natural selection. Today, there are those involved in trying to do that. Thirty seven years after he published his Gaia rebuttal, “Is Nature Really Motherly?”, Ford Doolittle of Dalhousie University, Nova Scotia, is working to “Darwinise” Gaia. The original hypothesis, he says on the phone, was “remarkably naive, the kind of thing you expect from an engineer – he had no evolutionary biology training, and he wouldn’t have come up with the hypothesis if he had. So it’s probably good that he didn’t.” These days, Doolittle is working to license Lovelock’s statements about the planet’s function on a biospheric level, in a way that would make sense to Darwin.

Master builder: Lovelock, pictured in the 1980s, now calls himself an engineer rather than a scientist. Homer Sykes / Alamy​

“Prior to Lovelock we tended to see the world as all about physics. He pointed out that the atmosphere and everything about the planet is profoundly influenced by the presence of life. It was a deep realisation and a very simple one. Sometimes the brightest people are the ones who come up with the deepest and simplest ideas, after which you think, ‘Duh, why didn’t I think of that?’”

Doolittle adds that Lovelock is someone “completely without guile”.

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Lovelock funded his Gaia research independently. Projects for Nasa in the Sixties allowed him to purchase his lab in Bowerchalke and Hewlett-Packard paid him a retainer to work on its gas chromatographs for 32 years. “It was perfectly possible for me to do science as a loner and not be bothered,” he says. He also worked for the secret service, “as a kind micro-Q” (he references the inventor in James Bond), though having signed the Official Secrets Act 80 years ago he won’t reveal his creations.

“You only need five sources of income to make a living,” he says. “Like any freelancer. Being inside an institution would have been a great loss, something I did not want.”

Most institutions, that is. In the Seventies, Nasa offered him the job of running the Viking mission to Mars. His wife Helen, mother of his four children, had developed multiple sclerosis – a condition she would live with until her death in 1989. Lovelock could not take the job because of the price of US health care. “The nurses who looked after Helen said I should be proud to be one of the only ones that stuck with their wife right through to the end of a disease like that.”

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His ideas, and his defence of them, rest on opinions that often set him directly against the world of science. When I last spoke to him a decade ago he described visiting the Hadley Centre – the government’s research base for climate change – and seeing one group of scientists working on the melting of Arctic ice, another on rainforests, another on deserts and droughts. “Each worked in their own little bailiwick and they couldn’t see the bigger picture. The whole situation looked a hell of a lot worse than they thought it was – any amateur could see that.”

Why has scientific research become so territorial? “Because of ambition. The universities are very much like the medieval church. They’ve divided everybody up into little segments so they have more control. It’s a purely personal thing so that different leaders can get their honours. It is no longer motivated by the search for knowledge.”

Universities often dismissed his scientific hunches anyway. The neglect of intuition in science comes up time and again in his new book, Novacene, where he describes the “non-linear, intuitive insight”, by which he claims to work. “Dynamic systems have a non-linear way of thinking,” he says, of the Earth. Some ideas lie beyond speech. These kind of statements make his ideas resonate for the casual reader while irritating the scientific world. How would he see intuition better employed in science?

“It is not an easy thing to explain,” he says. “I said I was an engineer – maybe I should have said I was an inventor. The key to invention is somebody saying, ‘I wish someone would think of a way of doing this thing.’ Then suddenly the how to do it comes into your head. Before that, you wouldn’t even have thought about the need to invent it. It brings all the bits together that are floating about in your brain and out comes the answer. That’s the nearest I can get to explaining how invention works.”

In 2012, in a move that seemed typically contrarian, he said he’d been “alarmist” about climate change. Yet talking to him now, you don’t get the sense he no longer thinks we are doomed – he has just moved the story on. In Novacene his sense of the future is almost psychedelic. He sees rubbish collected from his garden and wonders whether one day “the internet could serve the same purpose as these vans – take away redundant information and dispose of it in some vast unfathomed depth of the universe; huge transmitters cited at the poles, broadcasting junk mail and misinformation – what a splendid way to keep cool!”

He imagines animals grazing on solar-powered plants – batteries hanging like fruit from trees. “Well, why not?” he laughs.

When do these visions come to him? “I’ve never had any problems that way. Life has been one mass of visions, really.”

He still has big ideas of geo-engineering, telling Richard Branson that planes should carry sulphur dioxide to release at height, or mix with their fuel, to create an atmospheric sunshade. You’d need about 20 million tonnes of the stuff, he says – “not an awful lot” – to readjust the planet’s heating. Humans will give way to machines – and machines may put such things into place.

And us? Our role on the planet, he’s now sure, was to “convert sunlight into information” – to bring knowledge, to make those machines and make sense of it all. “It’s just part of evolution,” he says. “We don’t want to admit we are not the ultimate end-point.”

In Novacene Lovelock makes Gaia sound Darwinian, albeit with a robot twist. The Earth is not protecting human life, but life itself. He gets on well with Richard Dawkins (“we let our disciples battle it out”). He says that WD Hamilton finally spoke out in support of Gaia – “then went to Africa and died. They all swivelled round.”

The University of Exeter talks of “Gaia 2.0” – that human beings might still “upgrade the planetary operating system”. Lovelock, meanwhile, has long drawn parallels between his end-point and the age of man. We’ve had a good run, he says: “I do sometimes wonder whether the universe had the intention of vanishing into a cloud of intelligence.”

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At the end of his photograph album is a picture of a tanned toddler in a white frock. It is James Lovelock aged three, a year after his first memory – lying in the garden and suddenly realising he was alive. I ask him if his mother had any more children. He replies: “One was quite enough for her.” Born in Bermondsey, London, Nell Lovelock won a scholarship to grammar school but was prevented from taking it up by family poverty, and worked in a factory, sticking labels on pickle jars. “She was a person with a great deal of bitterness,” he says. “She became more and more left wing and feminist, which is understandable.”

On paper, Nell sounds inspirational – she was a socialist and a suffragist (“She believed it was a woman’s right to smoke,” he says “which was not good for me. Ours was a house full of tobacco smoke”). She was also forgiving of his school reports: “I give her 100 per cent marks for the fact that she never interfered. She said, you’re obviously intelligent, what does it matter what they say?”

“She dumped me on my grandmother,” he says; he spent his first few years in Letchworth Garden City while his parents ran their Brixton shop. During the First World War, Nell had worked as secretary to the clerk for Middlesex County Council where, attending tribunals, she observed that Quakers were exempt from national service. She enrolled her son in a Quaker Sunday school. Lovelock was a conscientious objector during the Second World War. “I don’t think I ever really had pacifist principles,” he says today, “except to please my mother.”

Yet a strange sense of not having been protected persists in him. Nell was anti-vaccine, he points out, and never gave him his smallpox jab. “The results of taking it in my twenties were violent.”

In Novacene, he describes his mother’s view of nature as Hardyesque – country life was brutal and unforgiving – while his father taught him to love the Earth. One wonders whether his interest in mother nature – wonderful, beneficent, ultimately dismissive of her children – was on any level formed by those early feelings.

In 1965, he told a group of Shell executives that by the year 2000, climate change would be their biggest problem. In 2008, he said there was nothing we could do about it. In 2014, he advised moving to more habitable bits of the Earth. Recycling, he told the Guardian, was “almost certainly a waste of time and energy”; a green lifestyle was “ostentatious grand gestures”. Our biggest sin was the atom bomb – because it put us off nuclear energy, the safest source. Pollution? He lived through Brixton in the Thirties!

Yet he remains a reluctant icon of the Greens (“the right thing to be, I think”). Caroline Lucas, leader of the party, acknowledges this complicated relationship, telling me, “Despite a number of significant policy differences I have with James Lovelock, I have enormous admiration for his groundbreaking work. No one can doubt his extraordinary contribution to environmental thought over many years, with the Gaia theory having shaped the thinking of so many in the Green movement.”

What does he think about small green gestures – like paying for plastic bags? “I don’t get indignant about it.” And air travel? “Far too many people travel. People don’t need to go on holiday as much as they do. It’s warming up here” – he points towards the beach. “Look at it.”

Are environmentalists wrong to be pessimistic? “I think they’re just ignorant. The total Earth’s climate system is unbelievably complex and understanding it will take an awfully long time for some very bright people, if they exist at all. The Greens should know better than to get tangled up in that. Politics is complicated enough.”

Of politicians he adds: “The most impressive thing is their ignorance. It really is deep. They’re convinced that if you have a windmill on the top of every house it’ll solve the whole problem. They believe everything they’re told by salesmen and the Greens!”

He once said that Christians and the Greens share an effective use of guilt. Religion – or a sense of its “inner core of truth” – floats at the margins of his conversation. The story of Adam and Eve is poignant, he thinks – punished for their knowledge with expulsion from the garden. His sense that we are a chosen species – developing by natural selection, creating intelligent life and then snuffing it – would seem to marry science and religion in his mind.

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Lovelock fell in love at the age of 69, with an American called Sandy Orchard whom he’d first met before his wife died, when she approached him to speak at a conference. For his 80th birthday, they walked hundreds of miles round the south coast on the path that passes his front window. It took 13 weeks. For his 100th, he spoke at Blenheim Palace, recalling his time on a Quaker farm, doing a milk round with a horse called Alan. The location was significant.

“Sandy and I fell in love at Blenheim,” he says. “I’d been to a meeting she had organised. Something clicked, I don’t know what. After dinner, I was feeling mellow and I went and had a pee. Just ahead of me was a group of four women talking fairly vigorously. Sandy was among them.

“Without a word from either of us, we just went straight into each other’s arms. Never said a word. And went down the steps. People talk far too much. On a matter like that, of falling in love, you should keep as quiet as possible.”

Kate Mossman is the New Statesman's features editor