The most scrutinised technology so far has been ocean fertilisation, which involves using iron to stimulate phytoplankton growth in the ocean, increasing the uptake of CO2.

One study, for example, has shown that about half of a phytoplankton (algae) bloom stimulated by iron sank to the deep sea, locking the carbon away on a potential timescale of centuries.

But another showed that little CO2 was taken up by the organisms and that the potential for iron fertilisation may depend strongly on the location where it is attempted.

And some schemes have attracted controversy: In July 2012, for example, 100 tonnes of iron sulphate was deposited into the Pacific Ocean, off Canada's west coast, in an attempt to help restore salmon stocks there. The move outraged environmentalists opposed to ocean fertilisation.

While the idea continues to have its adherents, John Shepherd from the National Oceanography Centre at the University of Southampton, UK, who also chaired the 2009 Royal Society report, is doubtful about the benefits.

"Ocean fertilisation involves huge interference with the ecosystem. You have a big environmental impact with a small desired side-effect."

Progress on other projects has been hindered by factors external to their intrinsic merits. Last year, a project known as Spice, which was to have deployed a tethered balloon to disperse water into the air - as a prelude to spraying climate-cooling sulphate particles - was grounded.

Core to the decision was a patent application lodged on some of the technology, though the team cited other concerns among the reasons for the postponement.

Some scientists point out that manipulating the climate in one part of the world could have consequences elsewhere. Therefore, the argument goes, any action of this sort would need to be on a global level with international agreement.

Changing another country's weather is even classed as a war crime under the Geneva Convention of 1976.

Paul Nightingale of the Science and Technology Policy Research department at Sussex University, UK, says there is currently no infrastructure in place for such decisions to be made about our global climate.

"As a consequence they will be extremely contested," he adds.

Rose Cairns, also from Sussex University, has written a report for the Economic and Social Research Council (ESRC) on the area. She says one issue is that geoengineering remains an extremely ambiguous term because the technology is so diverse.

Spraying aerosols into the stratosphere, for example, could be "highly controversial", while a global project to plant trees is likely to cause much less furore.

As with any new technology, unpredictable side-effects of geoengineering cannot be ruled out.

For example, in addition to any benefits it might have, it is thought that lacing the stratosphere with sulphate aerosols could deplete atmospheric ozone and exacerbate the risk of drought - particularly in Asia and Africa where it might adversely affect the monsoon.

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