James Lovelock was on his finest doom-mongering form when he spoke at the House of Commons last week. Humanity, he told MPs, would be lucky if just 20% of us survived imminent climatic disaster.

“We have passed the point of no return,” he insisted. “We have reached a point where civilisation itself is threatened and we have no one to blame but our own ignorance and greed.”

Buried among the gloom, however, was an unexpected message. If humanity has any hope at all, said Lovelock, it lies in the use of radical technologies to deal with the greenhouse gases that are altering our climate.

Lovelock is already renowned for supporting nuclear power as a way of cutting carbon emissions, but he is now proposing something even more radical: a global programme to pollute the stratosphere with sulphur compounds. These, he suggested, would enable it to reflect more sunlight back into space — and so 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 said.

Twenty years ago this suggestion would have sounded mad.

The great environmental battles then were to rein back sulphate pollutants which were damned as the cause of acid rain — and Lovelock’s own Gaia theory, which suggests that the Earth behaves like a living system, was one of the philosophical underpinnings for green activists.

What, then, is he doing suggesting a scheme that would effectively reengineer the behav-iour of the Earth itself? Could Lovelock be right that such ideas really do offer a new and practical way of dealing with climate change?

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, America’s space agency, to work out which of them held the most promise.

Their ideas were far-ranging and perhaps farfetched. 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 which 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 has the collective power to heat up the planet, albeit inadvertently, then perhaps it might also find a way of cooling it down.

“Geo-engineering, as we call it, remains a last resort but it could save the world,” said Roger Angel, professor of astronomy at Arizona University, who suggested 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.”

Angel, whose research is supported by Nasa, has proposed launching a cloud of tens of thousands of spacecraft, each about 2ft-3ft across, to the inner Lagrange point.

This is a point between the Earth and the sun where gravitational forces cancel out, allowing such satellites to maintain a constant position.

“This would cut out just under 2% of the solar radiation reaching Earth for a few trillion dollars — less than 0.5% of the world’s GDP,” he said.

“If Greenland starts to melt then the damage from rising sea levels will be far greater than that. Such projects could be a very good investment.”

It is a view shared by Paul Crutzen, former professor of ocean science at the Scripps Institution of Oceanography, California, whose life’s work has been fighting pollution. Now he wants to reverse that to tackle the far bigger threat from climate change.

Back in 1995 Crutzen won the Nobel prize for research into the workings of the ozone layer, the part of the atmosphere that screens out deadly ultraviolet radiation.

It was his work that helped to prompt governments to block the use of chlorofluorocarbons and other pollutants, including sulphates, that were attacking the ozone layer.

However, in 1991 a massive volcanic eruption from Mount Pinatubo in the Philippines saw about 10m tons of sulphur being ejected into the stratosphere.

Over the next two years global temperatures fell by about 0.6C — comparable with the 0.7C rise attributed to greenhouse gases.

Crutzen now believes that humanity could save itself from climate change by replicating the effects of such an eruption. He suggests launching hundreds of rockets laden with burning sulphur into the stratosphere.

Crutzen realises the ironies involved in such a programme. Sulphur is not only the cause of acid rain, but also attacks the ozone layer that he has spent his career protecting.

“We want to clean up the environment because pollution is unhealthy,” he said recently, “but this pollution also cools the Earth by reflecting solar radiation into space.”

Perhaps the simplest idea for cutting the amount of sunlight reaching the Earth comes from Stephen Salter, professor of engineering design at Edinburgh

University. Salter, a renowned marine engineer, has drawn up designs for a fleet of yachts that would pump fine particles of sea water into the air. These would create clouds that would then reflect the sun’s rays back into space.

“Our calculations suggest that increasing the amount of light reflected by just 3% would be enough to counteract the global warming caused by greenhouse gases,” he said.

“Worldwide about 1,000 vessels would be needed to spray about five tons of water a second into the air. We could station them off Greenland in the northern hemisphere’s summer and then float them down to Antarctica for its hot season.”

The beauty of Salter’s boats is that they would be powered entirely by wind but with conventional sails replaced by Flettner rotors — vertical cylinders that rotate as the wind passes over them.

“It might seem a farfetched solution, but when the planet is in such a dire situation we have to examine every option,” Salter said.

However good such schemes might be at slowing down climate change, they would not prevent the other great threat posed by greenhouse gases: the acidification of the oceans.

About half of the 29 billion tons of carbon dioxide released by human activity each year becomes absorbed by sea water. This saves the Earth from warming even faster, but it is making the seas increasingly acid and threatens to wipe out many forms of marine life.

A number of geoscientists are proposing systems that can strip the surplus CO2 out of the air, which would ease ocean acidification.

Professor Klaus Lackner of the Earth Institute at Columbia University has designed a 100ft tall artificial “tree” that could strip 90,000 tons of CO2 from the air each year — equivalent to the output of 20,000 cars. Lackner believes that planting forests of such devices around the world could halt the surge in levels. CO2 Lackner’s “trees” would work by passing air through filters containing sodium hydroxide. This would react with CO2 to create a solution of sodium carbonate that could be piped away. Eventually the CO2 collected would be stored away for ever, perhaps in vast reservoirs under the seabed.

Such massive engineering programmes raise all kinds of problems. How would they be constructed and powered without generating more carbon dioxide? How much land would the artificial forests take up?

Other researchers are seeking more natural solutions.

Most of these focus on exploiting the tiny marine algae that fill the upper layers of the world’s oceans. For aeons these have been part of nature’s way of reprocessing CO2. When they photosynthesise, die and fall to the seabed they remove billions of tons of carbon dioxide from the seas each year.

In the past few decades, however, humanity’s outpour-ings of CO2 have swamped this system. The algae can no longer keep up.

Craig Venter, the maverick American scientist who helped to crack the human genome, believes that there could be ways to accelerate the rate at which such plankton absorb CO2 In a recent ocean expedition he collected thousands of new species. Their genomes are undergoing analysis to identify those with a high ability to collect carbon.

Venter envisages creating vast colonies of bacteria and other microbes that are able to scrub the atmosphere of greenhouse gases such as carbon dioxide and methane and perhaps even convert the pollutants to ethanol, which can be used as a fuel.

“Microbes are responsible for the creation and maintenance of the Earth’s atmosphere,” he said. “It is important to understand the role and function of these organisms to ensure the survival of the planet and human life on it. It’s clear we’ve only begun to scratch the surface of understanding the microbial world.”

What stands out most about such ideas is not so much their practicality as the fact that most of them originate in America, where politicians and the public remain strongly resistant to cutting their carbon emissions.

Europe, by contrast, has taken the line that the best way to deal with climate change is by trying to slash the greenhouse gas emissions that cause it.

Critics of the geo-engineering approach believe that it is driven largely by American politics rather than by pure science.

“Schemes like reflective satellites or airliners pouring out sulphates sound interesting, but in reality they are just ludicrous,” said David Lee, professor of atmospheric chemistry at Man-chester Metropolitan University.

“The Americans are desperate to find technological fixes because they hate the idea of cutting back on their high-carbon lifestyles. We should listen to all new ideas, but in the long run cutting carbon remains the best answer.”

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