A giant mirror drifts slowly through space between the earth's surface and the sun, intercepting the rays of sunlight before they reach the earth and deflecting them safely away.
The mirror, made of millions of silicon chips, is situated at a point in space where the sun's gravity and the earth's cancel each other out. This vast structure, assembled painstakingly for years by spacecraft, drifts naturally away from its starting point over time, but complex on-board systems nudge it gradually back to resume its vital role in keeping us safe.
This space mirror is - so far - science fiction. Such a structure would cost hundreds of billions of dollars, even if it were technically feasible. But soon many scientists say we may need to start building space mirrors, creating artificial clouds, or altering the chemistry of the sea to prevent the worst effects of global warming.
Climate change is occurring faster than predicted and the risks are growing day by day. Altering the earth's systems to help cool the planet may soon be the only option, many scientists believe, as our runaway appetite for fossil fuels overtakes our good intentions on emissions.
"The chances of reducing emissions fast enough now are very low," said Stephen Salter, professor of engineering design at Edinburgh University. "This is a jolly strong reason to look at alternatives."
The science of altering the world's natural systems is called geo-engineering. Once on the whacky fringes of scientific research, the subject is rapidly becoming mainstream . John Holdren, chief scientific adviser to US president Barack Obama, said in public a few weeks ago: "It's got to be looked at . . . We don't have the luxury of ruling [out] any approach."
The debate has been intensified by a set of studies published last week in the journal Nature. They concluded the world had little chance of holding temperature rises to 2°C - a level widely regarded by scientists as the limit of safety, beyond which climate change becomes irreversible and potentially catastrophic. Such a level of warming risks leading, for example, to the melting of permafrost in Siberia, so releasing large quantities of methane, and in turn causing stronger and more rapid warming.
Most suggestions for geo-engineering fall into one of three categories. Some of the most outlandish would block out the sun's rays using mechanical means - a sunshade or mirror, for instance. These would be enormously expensive, even if possible. Some of the most promising proposals involve ways to increase the earth's ability to reflect sunlight back into space. Sulphur particles shot into the stratosphere, could reflect enough sunlight to make a measurable difference. But sulphur causes acid rain, although proponents point out much less sulphur would be needed than pours from power stations.
A cheaper method would be to spray seawater into the air from boats. This creates clouds made up of smaller- than-usual droplets of water, which reflect more light. Prof Salter says a drawback would be that, as well as reflecting sunlight, clouds trap infrared heat. But he says the net benefit would be huge - each drop would reflect 20bn times the amount of energy used to make it. He estimates that a about 500 ships would be needed.
Scientists are also exploring removing carbon from the atmosphere. One way is to fertilise the oceans with iron, so plankton grows and absorbs carbon. Less realistic are enormous air "scrubbers" - banks of sails coated with chemicals that react with CO 2 . Two unsolved problems are mastering the chemical absorption and the sheer amount of energy that would be needed to propel vast volumes of air.
Employing geo-engineering would not remove the need for deep emissions cuts, Prof Salter warned: "No one in geo-engineering would argue that." The two must be pursued in tandem, or rising emissions would counteract the benefits of the earth-altering projects.
Tim Lenton, professor of earth system science at the University of East Anglia, who conducted a review of geo-engineering methods, said scientists must beware of "tinkering with a system you do not fully understand". For instance, he said, studies showed that putting sulphate aerosols into the atmosphere caused drying in vulnerable regions.
By Fiona Harvey, Environment Correspondent
Published: May 9 2009 03:00