American researchers have discovered that the amount of water high in the atmosphere is far more influential on world temperatures than previously thought.
Although the findings do not challenge the theory of man-made global warming, they help explain why temperatures can rise and fall so dramatically from decade to decade.
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Water vapour has long been recognised as an important greenhouse gas. Like methane and carbon dioxide, it absorbs heat from the sun that would otherwise be reflected back into space, keeping the planet warm.
However, most computer models that predict climate concentrate on the levels of water lower down in the atmosphere.
Dr Susan Solomon, of the US National Oceanic and Atmospheric Administration, said: 'Current climate models do a remarkable job on water vapour near the surface.
'But this is different — it’s a thin wedge of the upper atmosphere that packs a wallop from one decade to the next in a way we didn’t expect.'
Observations from weather balloons and satellites show that 'stratospheric water vapour' increased in the 1980s and 1990s and dropped after 2000.
The changes took place in a narrow altitude region of the atmosphere where they would have the biggest impact on climate.
The reasons why water vapour rises and falls remain a mystery, the scientists say. However, the study estimates that the drop in water vapour since 2000 caused surface temperatures to rise 25 per cent more slowly than they would have done otherwise.
And the increase in stratospheric water vapour in the 1990s is likely to have accelerated the rate of global warming by around 30 per cent, the scientists say.
The stratosphere is a region of the atmosphere from about eight to 30 miles above the Earth’s surface. Water vapour enters the stratosphere mainly as air rises in the tropics.
Dave Britton from the Met Office said the study highlighted the complexity of climate science. 'But it does not challenge the basic science that carbon dioxide and other greenhouse gases released from human activity are warming the planet,' he said.
Dr Vicky Pope, head of climate science at the Met Office, said: 'Whatever's causing this change from decade to decade is having an influence at the surface. But it is a small variation on top of the long term increase in manmade greenhouse gases.'
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