THE threat of devastating climate change is now so great that some scientists say it is time to investigate a Plan B - geo-engineering on a planetary scale.

Such methods of altering the world’s climate may become necessary, they say, unless emissions of greenhouse gases fall within five years.

Ideas that were once the realm of science fiction - such as creating artificial trees to absorb carbon dioxide, or reflecting sunlight away from the Earth - are coming under serious scrutiny as temperatures and CO2 emissions continue to rise. The issue has become so pressing that the Royal Society, Britain’s national academy of science, is preparing a report on the feasibility of geo-engineering.

Professor John Shepherd, chairman of the working group, said: “Our study aims to separate the science from the science fiction and offer recommendations on which options deserve serious consideration.”

The report is not yet complete but the personal view of Professor Brian Launder, one of its contributors, is that without CO2 reductions or geo-engineering, “civilisa-tion as we know it will end within our grandchildren’s lifetime”.

At present, global emissions of greenhouse gases are still rising by 2% to 3% a year, according to the Met Office. If that continues, average world temperatures are projected to rise by as much as 5.5C by 2100.

Launder, professor of mechanical engineering at Manchester University, reckons that extracting carbon from the atmosphere would be too slow a process to prevent significant warming. In his view “the only rational scheme is to reduce the amount of sunlight reaching Earth and to reflect back more of it”.

One method under detailed analysis is to make clouds brighter – especially in the Pacific where the ocean temperature has great influence on world climate. “If these clouds can be brightened so you increase the sunlight reflected even by a couple of per cent, it looks as though that could be enough . . . to prevent most of the effects of global warming,” said Launder.

Professor Stephen Salter of Edinburgh University is investigating how ships could spray droplets of sea water into the atmosphere where they would evaporate, leaving tiny salt crystals to rise on air currents into the clouds.

The crystals would act as “nuclei” around which water vapour could condense and thus increase the reflective power of the clouds, bouncing more of the sun’s energy back into space. But critics warn that although such schemes might lower temperatures swiftly, they would have to be maintained for long periods and the side-effects are unknown.

Dr Vicky Pope, head of climate change advice at the Met Office, said: “Anything that alters the climate in a different way from reducing carbon has inherent dangers because we don’t understand the climate well enough.”

For this reason Professor Tim Lenton, another climate scientist, prefers technologies that could suck CO2 out of the atmosphere: “We should push for the strongest mitigation [of CO2 emissions] possible. But among the geo-engineering options there are some that might be useful add-ons.”

One idea is to create plantations of fast-growing trees such as willow and turn them into “biochar”. Plants grow by extracting CO2 from the air and converting it to wood, so the idea would be to turn the wood into charcoal, using giant ovens. Then it would be buried so the carbon could never be released back into the air.

Other experts warn that geo-engineer-ing risks diverting attention from the need to create a carbon-neutral economy.

“Wind and solar energy are at least in the pipeline, whereas geo-engineering is much more speculative,” Pope said.

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