From mirrors1,000km across to supersize kitchen utensils that fling salt spray into the air, Fred Pearce provides an overview of engineering solutions to cooling off the planet as global warming becomes a burning issue.

They want to save the planet. The engineers who in past decades brought us cloud seeding and plans to warm Siberia by melting the Arctic icecap now say they can stave off global warming by shading the planet with tiny metal balloons or erecting giant egg whisks in the oceans.

Until recently, planetary geo-engineers were at the outer fringes of climate science. But mainstream scientists are becoming so depressed by the failure of politicians to cut greenhouse gas emissions that they are starting to take a serious interest. They fear that one day we may need a climate fix very quickly, especially if climate change happens more rapidly than mainstream models predict and global warming needs to be halted within months rather than decades.

There are basically two approaches on offer. The first, "scrubbing" flue gases, involves capturing carbon dioxide as it goes up power station chimneys, then storing it out of harm's way in geological structures such as salt domes and old oil wells or even in deep-sea sediments where high pressure and low temperatures would immobilise it.

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This idea of carbon sequestration has recently slipped into the mainstream. This year, BP launched a pilot project. And Sir David King, the British Government's Chief Scientific Adviser, says that by 2020 Britain could be burying a quarter of its power station emissions in old oil fields beneath the North Sea. Recent studies suggest there is ample space for several centuries' worth. Many argue that if countries such as India and China are bent on exploiting huge coal reserves in their drive for industrialisation, the development of this technology is vital.

The second approach seemed firmly in the realms of science fiction until Dutch chemist Paul Crutzen, winner of a Nobel prize for helping unravel the chemistry of ozone in the atmosphere, put it on the agenda this summer. This idea is to allow carbon dioxide to accumulate in the atmosphere but to put the brakes on warming by shading the Earth's surface from the Sun.

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Ideas on how to erect a parasol above the planet have a long, imaginative and not altogether savoury history. In the mid-1960s, US scientists first discussed meeting any future global warming by floating billions of small reflecting objects, such as white golf balls, across the tropical oceans - thus increasing the reflectivity of the planet.

In 1982, Soviet climatologist Mikhail Budyko suggested instead throwing light-reflecting sulphate particles into the stratosphere. He reckoned that 10 million tonnes of sulphate sprayed from high-flying aircraft each year could scatter 1 per cent of solar radiation and counter a doubling of carbon dioxide levels in the atmosphere.

Crutzen revived this idea, arguing that a mere 5 million tonnes of sulphate a year would do the job - in essence, mimicking the cooling effect of a major volcanic eruption. In 1815, the eruption of Mount Tambora caused a "year with no summer". In future, every year could have no summer.

Tom Wigley, a leading climate scientist of the US Government's National Center for Atmospheric Research in Boulder, Colorado, supports Crutzen. "Until recently, these radical ideas have been dismissed - and probably rightly so. But now it's come to the point where I think we have to consider these things seriously."

One problem with this approach is that the sulphate particles would last only a few years before returning to Earth. Meanwhile, carbon dioxide, which lasts centuries, would keep accumulating. So, to stabilise temperatures while continuing to emit carbon dioxide, the world would have to constantly increase the amount of sulphate deployed. And the implications for the planet of stopping, even for a few months, would become ever more dire.

Others have suggested putting more permanent objects in the stratosphere. Shortly before his death in 2003, Edward Teller of the Lawrence Livermore National Laboratory in California (better known as the father of the US hydrogen bomb) proposed putting shards of metal, or "optical resonant scatterers", that would reflect light of particular wavelengths.

His colleague Lowell Wood says that a million tonnes of tiny aluminium balloons, about 4mm across and filled with hydrogen, could do the same job for as little as $1 billion a year. Failing that, he says, future Space Shuttle technologists could assemble a diaphanous mirror 1,000km across and park it between Sun and Earth to deflect solar radiation. Positioned at the right orbit, just 3,000 tonnes of mirror could compensate for a doubling of carbon dioxide levels. It could cost hundreds of billions of dollars but would be a more permanent solution.

If aluminium balloons or space mirrors seem too far-fetched, Wigley's colleague John Latham suggests a more down-to-earth option: making clouds whiter. This is a variation on the old idea of "seeding" clouds.

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Clouds form when water vapour gathers around tiny hard particles in the air, such as salt spray, to form droplets. To seed clouds and make it rain, you can add artificial particles. But if you add too many, you get too many droplets, none of which grows big enough to fall as rain. Instead, the clouds get whiter. Latham's idea is to deliberately add too many particles, packing clouds with droplets to boost their reflectivity. And because they do not rain out, it is likely they would last longer too.

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Latham calculates that doubling the number of droplets in clouds above the oceans would shut down several decades of global warming. He proposes creating the particles by deploying 70m-high wind-powered machines across the oceans to fling salt spray into the air. His collaborator, Stephen Salter of Edinburgh University, has created a prototype "spray turbine" that looks rather like a giant egg whisk. But, like injecting sulphates into the stratosphere, the enterprise would need to be huge and constant.

Practicality is not the only problem, however. Tinkering with something as crucial and poorly understood as solar radiation could have dramatic consequences for our climate and life-support systems. There is, for a start, the small matter of the colour of the sky. Wood's optical chaff or Crutzen's sulphate aerosols would alter the quality of sunlight, making the sky less blue. More seriously, the combination of less solar radiation penetrating the lower atmosphere and more of that radiation being trapped by greenhouse gases when it gets there would change the temperature gradient of the lower atmosphere. One likely consequence, says Peter Liss of the University of East Anglia, is that the stratosphere would cool, creating ideal conditions for the rapid destruction of the ozone layer.

Uniquely, manipulation of the planet's reflectivity would give humanity the chance to decouple planetary temperatures from carbon dioxide levels. In theory, we could have carbon dioxide levels soaring above current levels while global temperatures are held in check.

Wood sees that as a huge bonus. More carbon dioxide in the air would make plants grow faster, he says. Poor tropical farmers would get bumper crops. Famine would be prevented at a fraction of the cost of cutting carbon dioxide emissions.

Not everyone agrees. There is some evidence that changing the intensity of solar radiation has a direct influence on rainfall in the world's drought-prone regions. Wood's carbon dioxide fertilisation would not work if the rains failed.

Perhaps worse, the accumulating carbon dioxide would put into overdrive the acidification of the oceans. Coral and shells would simply dissolve. This would not only be a catastrophe for fisheries, but it would also reduce the ability of the ocean to absorb carbon dioxide, raising levels yet further.

Could geo-engineering really solve our climate problems? Environmentalist James Lovelock says: "I am one of the minority of greens who think the cure for man-made climate change lies in engineering, not in abandoning it." He thinks that being a planetary manipulator is something akin to being a doctor. He wants "a Hippocratic oath for geo-engineering".

But Ed Boyle, an oceanographer at the Massachusetts Institute of Technology, says he finds the idea of "monkeying around with a system as complex and poorly understood as the Earth's climate rather appalling".

And even if these technologies were to work according to plan, and if the Earth system did not bite back with some unanticipated feedback, we would still face the daunting prospect of what to do with the control they gave us over Earth's climate.

Once built, technologies such as solar scatterers would be fearsome weapons in the hands of a global superpower, as well as being the terrorist target of all time. Whose hand would be on the global thermostat?

Fred Pearce is an environment consultant and journalist. He is author of When the Rivers Run Dry , Eden Books, ?18.99.

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