Now in his 90th year, and still dauntlessly proclaiming unpalatable truths, James Lovelock is the closest thing we have to an Old Testament prophet, though his deity is not Jehovah but Gaia, his concept of planet earth. In his latest book, The Vanishing Face of Gaia, he warns, as he has before, that global warming is probably irreversible, and that once a tipping point is reached change to a new climate may be rapid. The earth's landmasses will be largely destroyed by flood and drought, and most of the world's seven billion inhabitants will not survive. All this should make for a bleak read, yet Lovelock writes with such challenge that the effect is strangely exhilarating.

It seems from John and Mary Gribbin's absorbing account of his life, He Knew He Was Right, that he owes his toughness and confidence partly to his parents, who were working-class Londoners with an ardent belief in education. Nell, his mother, won a scholarship to Islington grammar school but was too poor to take it up and started work at 13 in a pickle factory. She became an early feminist, with a passion for classical music, and held down a responsible job with Middlesex county council during the first world war. His father Tom served six months hard labour as a teenager for poaching, then got a job in Vauxhall gasworks until his employer realised he was illiterate and sent him to be educated at Battersea polytechnic. He never looked back, developed a love of art, and opened a shop selling paintings in Brixton. While his parents fostered Lovelock's individualism, his educators implanted a permanent dislike of authority. At Brixton grammar school he was repeatedly punished for refusing to do homework or compulsory games. Instead, he devoured science fiction, especially HG Wells, whose stories of global disaster have perhaps affected his own thinking.

He could not afford to go to university, which was, he believes, an advantage, because it prevented him becoming a specialist in a single scientific field, whereas to develop Gaia theory he needed to see the earth as an integrated system of chemical, biological and geological forces. His first job was with a photography firm, and in the evenings he studied chemistry at Birkbeck College. A keen hiker, he resented landowners blocking public footpaths with gates, so would cycle out at weekends to blow them up with home-made explosives. One barrier he destroyed belonged, he recalls, to the foreign secretary (he does not say which, but the dates suggest it was either Anthony Eden or Viscount Halifax). He did well enough at Birkbeck to be accepted at Manchester University, but could pay for only two years of the three-year course. For some weeks he lived solely on baked beans, and contracted scurvy. When war came he registered as a conscientious objector, and, having temporarily converted to Quakerism, worked on a Quaker farm. But his Manchester professor recommended him to the National Institute for Medical Research (NIMR), where he explored ways of shielding soldiers from burns. He refused to use the shaved and anaesthetised rabbits that were provided as potential burn victims, and instead exposed his own skin to heat radiation (“exquisitely painful”).

In other matters he was less squeamish. The laboratory kept stores of human blood for experiments, which was tipped away after 30 days. To Lovelock this seemed wasteful at a time of food rationing, so he took some home and tried mixing it with dried egg for omelettes (“quite palatable”). A similar scientific detachment apparently surrounded his marriage in 1942 to Helen Hyslop, the NIMR receptionist. It was, he says, arranged by friends who worried about his health and thought he needed a stable home. According to the Gribbins it was “a very British type of marriage”, where the couple remained “just good friends”. However, they produced four children and lived together until 1989 when Helen died of multiple sclerosis.

In 1948, working on the common cold at the NIMR, Lovelock invented a machine that used radioactivity to measure air currents. Typically careless of personal risk (his hobbies included mountaineering), he got the radium he needed by scraping luminous paint off war-surplus aircraft parts. Refining this prototype during the 1950s, he developed instruments sensitive enough to detect minute traces of pesticides in the environment and pollutants from spray cans and refrigerators in the atmosphere. These devices opened up a new era in ecosystem analysis, leading to the discovery of the ozone hole over Antarctica, and inaugurating the green movement.

His breakthroughs brought almost no personal gain. The instrument for detecting pollution was perfected during a year at Harvard in 1958-59, so the American government seized the patent, and Harvard, having promised him a fee, discovered it was not permitted to pay aliens. To keep himself and his family he sold his own blood. Fortunately he has a rare blood group and earned $50 a pint. The idea of Gaia came to him in 1965 when he was working at the Jet Propulsion Laboratory in Pasadena. The JPL plan was to send a spacecraft to Mars to search for signs of life, but Lovelock told them it was a waste of effort, all you had to do was analyse the Martian atmosphere. Sure enough, French astronomers using infrared instruments found that Mars had a stable atmosphere, almost entirely carbon dioxide, and comparing this dead state to the oxygen-rich earth convinced Lovelock that earth's atmosphere was created by the living things on it, and that our planet could be regarded as a single, self-regulating organism. The name Gaia was supplied by William Golding, his pub companion in the Wiltshire village where they both lived.

It was a mixed blessing. Scientists derided the name's earth-goddess suggestions, and in 1982 Richard Dawkins trashed Lovelock's first book, pointing out that it contradicted evolution theory. He adjusted his argument to meet Dawkins's criticisms, but almost as annoying were the hordes of new agers who flocked to Gaia's banner. Lovelock continues to distance himself from them in his new book, advocating nuclear fission as our only viable energy source, and dismissing biofuels and wind power as useless, or worse. The dangers of nuclear energy have been cynically exaggerated, he believes, by oil- and gasproducing nations for purely selfish ends.

With his grandchildren, his American wife Sandy, and his 35 acres of woodland, wild orchids and badger setts “given back to Gaia”, he emerges as a happy, kindly man. Yet he does not always seem fully alive to the horrors that must ensue when the starving, terror-crazed masses of climate refugees arrive in the earth's few remaining “lifeboats”, one of which, he believes, will be Britain. Observing that “genocide by tribal mobs is as natural as breathing”, and anticipating a “massive natural cull of humanity”, he seems to hark back to the scientifically detached Lovelock calmly eating his blood omelette. Yet he is still imagining ways to save civilisation - synthetic foodstuffs; a carbon-fibre disc orbiting the sun to protect us from its heat - and he has been allocated a seat on Richard Branson's Virgin Galactic, so that he may at last see Gaia from space. Let us hope she is still habitable when he gets back.

The Vanishing Face of Gaia by James Lovelock
Allen Lane £20 pp192
He Knew He Was Right by John and Mary Gribbin
Allen Lane £20 pp256