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Why has global warming paused? Water vapor may be in the answer.

A decline in stratospheric water vapor between 2000 and 2009 followed an apparent increase between 1980 and 2000, a team of scientists has found. That finding may have implications for global warming.

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Labourers look out of a steam train transporting coal at a power plant in Shenyang, Liaoning province January 6. A decline in stratospheric water vapor and increase in sulfate aerosols from the rising number of coal-fired power plants in China may explain a decade-long plateau in global warming.

Sheng Li/Reuters

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A decade-long plateau in global warming appears to have occurred in large part because the stratosphere – the layer of atmosphere that few but airliners enter – got drier.

That’s an explanation by a team of atmospheric scientists from the United States and Germany. They’ve studied trends in stratospheric water vapor over the past 30 years and calculated the effects of those trends on temperatures.

A decline in stratospheric water vapor between 2000 and 2009 followed an apparent increase between 1980 and 2000, according to balloon and satellite measurements that the team used. The decline slowed the long-term growth in global average temperatures by some 25 percent, compared with the warming one could expect from rising concentrations of greenhouse gases alone, the team estimates.

"There's not a lot of water in the stratosphere. It's extremely dry," says Susan Solomon, an atmospheric scientist at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colo., who led the team. "But it packs a wallop" in terms of its climatic effects, she says.

Other factors probably played a role as well in the temperature plateau, the team acknowledges.

Another contributor could have been sulfate aerosols from the rising number of coal-fired power plants in China, point out researchers such as Drew Shindell, with the National Aeronautics and Space Administration’s Goddard Institute for Space Studies (GISS) in New York.

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