At low altitudes they help to cool the Earth by reflecting sunlight away from it.

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But there, right at eye level, stand the tops of the 80-storey bridge towers, with skyscrapers reflecting sunlight in the distance.

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Another, which would oppose warming, is a rise in the quantity of sulphate-based aerosols, which encourage cloud formation and thus cool the air by reflecting sunlight back into space.

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However, after three years of observations of low stratus, cumulus and stratocumulus clouds over land, Anthony Del Genio of NASA's Goddard Institute for Space Studies discovered that when air temperatures were higher, clouds were thinner and thus less capable of reflecting sunlight.

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Scientists have known for a long time that aerosols cool climate by reflecting sunlight and making clouds brighter, but the new study is the first to use a global model to estimate the net effects on plant carbon intake resulting from this type of atmospheric pollution.

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Pines rustle gently in the breeze and droplets of dew sparkle on the fields, reflecting watery sunlight filtering through the clouds.

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For one thing, we're going to need to cool the planet somehow, probably by reflecting some sunlight back into space.

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Injecting sulphate particles into the stratosphere, for instance, would cool the Earth by reflecting more sunlight into space. (Nature has already shown that this concept can work, since volcanic eruptions that send sulphur-rich plumes into the stratosphere can temporarily alter the world's climate.) However, Dr Lenton notes that the method becomes less effective as the atmosphere becomes more saturated with particles.

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Then there are others who have proposed radical geoengineering schemes, such as injecting tons of sunlight-reflecting particulates into the stratosphere as a way to address the threat of climate change.

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Runaway climate change could spur an island nation doomed to drown under rising seas or a wealthy entrepreneur with do-good intentions to deliberately inject sunlight-reflecting particles high into the stratosphere in a bid to cool the planet, the World Economic Forum warns.

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