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03 December 2010

Goodbye grey skies, hello extra warming

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LOW, grey clouds help keep the planet cool. But as the world warms they will shrink and temperatures will rise ever higher, according to a study that could help to resolve one of the biggest uncertainties in climate science

As more greenhouse gases enter the atmosphere, more heat is trapped and temperatures go up - but by how much? The best estimates say that if the amount of carbon dioxide in the atmosphere doubles, temperatures will rise by between 2 and 4.5 °C.

That's a big uncertainty for such an important number, and clouds are largely to blame. High-level clouds trap heat, but those at low levels reflect sunlight and cool the planet. So depending on how they change, clouds could push temperatures up or down.

To find out, Axel Lauer of the University of Hawaii at Manoa in Honolulu and his colleagues used a relatively new climate model called the International Pacific Research Center Regional Atmospheric Model (iRAM). The model covers the eastern Pacific and parts of South America, in greater detail than global models. They focused on low-level marine clouds in the eastern Pacific, which have proved particularly tricky to model.

The researchers first tested iRAM's ability to simulate cloud cover between 1997 and 2008, and found it reproduced the amount and pattern of cloud cover and how it changed from year to year. Sixteen other climate models did a poorer job of this. They then ran iRAM for possible scenarios for 2090 to 99. With higher CO2 levels and temperatures, low-level cloud shrank by up to 10 per cent, letting in even more solar radiation (Journal of Climate, vol 23, p 5844).


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