Buried in the gloom, however, is an unexpected message. Lovelock is already renowned for supporting nuclear power as a way of cutting carbon emissions, but he is proposing something even more radical: a program to pollute the stratosphere with sulphur compounds. These, he suggests, will enable it to reflect more sunlight back into space and cool the world.

"We could fit sulphur dispensers on to the world's airliners so that they could mitigate the world's carbon emissions as they go," he says. Twenty years ago this suggestion would have sounded mad. The great environmental battles then were to rein back sulphate pollutants that were damned as the cause of acid rain, and Lovelock's Gaia theory, which suggests that the Earth behaves like a living system, was one of the philosophical underpinnings for green activists.

Yet an increasing number of climate researchers share Lovelock's vision.

Last November hundreds of them met at a conference organised by the Carnegie Institution and NASA, the US space agency, to work out which of them held the most promise. Their ideas were far-ranging and perhaps far-fetched. One scientist proposed launching a swarm of reflective spacecraft to hover between the sun and the Earth, blocking out some sunlight. Another suggested a fleet of large yachts that would spray seawater into the air, creating extra clouds to reflect sunlight back into space.

Underlying them all, however, was a single idea: that if humanity had the collective power to heat up the planet, albeit inadvertently, then it might also find a way of cooling it.

"Geo-engineering, as we call it, remains a last resort but it could save the world," says Roger Angel, professor of astronomy at Arizona University, who suggests the reflective spaceships. "The ideal solution is for humanity to cut its carbon emissions and stop heating the world in the first place. But if we get to the point where the world is warming by, say, 2C, then we should consider these other measures, however extreme they may seem."

The Sunday Times