The verdict by Britain's prestigious Royal Society came little more than three months before a UN showdown in Copenhagen on how to reduce the carbon emissions that drive climate change.

John Shepherd, a professor at Britain's University of Southampton, who chaired a 12-member panel which assessed the evidence, said geo-engineering was filling a perilous political void.

"Our research found that some geo-engineering techniques could have serious unintended and detrimental effects on many people and ecosystems -- yet we are still failing to take the only action that will prevent us from having to rely on them," he said.

The report cautiously said some geo-engineering schemes were technically feasible but were shadowed by safety worries and doubts about affordability.

Provided these questions were answered, such projects could be a useful tool as part of a worldwide switch to a low-carbon economy, it said.

But, the report warned, other geo-engineering schemes are so costly or so freighted with risk and unknowns that they should only be considered a last-ditch fix.

Just five years ago, geo-engineering was widely dismissed by mainstream climate scientists as quirky or delusional. As recently as 2007, the UN's Intergovernmental Panel on Climate Change (IPCC) cautioned of its potential risk and unquantified cost.

But the schemes are now getting a serious hearing in many quarters, helped by mounting evidence that climate change is advancing faster than thought while progress towards a carbon-curbing UN treaty is moving at glacial speed.

Supporters say geo-engineering can buy time to let politicians hammer out a deal or wean the global economy off polluting fossil fuels.

The report, "Geoengineering the climate: Science, governance and uncertainty," was based mainly on peer-reviewed literature.

It took a year to carry out, and the Royal Society came under fire from green groups who accused it of handing a cloak of respectability to a once-mocked scientific fringe.

The authors said geo-engineering fell into two main categories.

The most promising entails removal of carbon dioxide, such as by planting forests and building towers that would capture CO2 from the air.

Some of these projects could be harnessed alongside conventional methods to reduce emissions once they are demonstrated to be "safe, effective, sustainable and affordable," said the report.

The other category is called solar radiation management.

Instead of tackling CO2, it would act like a thermostat, turning down the heat that reaches Earth from the Sun.

Concepts in this field include deflecting the Sun's heat away from the Earth through space mirrors, scattering light-coloured particles in the high atmosphere to reflect the solar rays and using ships to spray water that would create reflective low-altitude clouds.

The advantage would be to lower temperatures quickly and could be tempting if global warming suddenly cranked up a gear, the report said.

But these techniques would not curb CO2 emissions that cause dangerous ocean acidification; their costs are unclear but possibly astronomical; and they may end up generating disasters of their own.

Even so, they should not be dismissed out of hand, given their potential in an emergency, said Ken Caldeira, a professor of climate modelling at Stanford University, California.

"We need to think if Greenland were to be sliding into the sea rapidly, causing rapid sea-level rise, or if methane started to de-gas rapidly from the Siberian permafrost, or if rainfall patterns were to shift in such a way that wide-spread famines were induced," he said.

"We would be remiss if we did not do what we could do to understand the potential of these options as well as their uncertainties and risks ahead of time."

Painting roofs white to reflect solar rays -- an idea gaining ground in California and other sunny places -- would provide only limited, local cooling and not affect the rise in global temperature.

"None of the geo-engineering technologies so far suggested is a magic bullet and all have risks and uncertainties associated with them," Shepherd said.

The panel called for funding of around 100 million pounds (162 million dollars) a year to kickstart research into the feasibility of geo-engineering schemes could be feasible -- and, if so, in what circumstances they should be applied and how they would be managed.

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