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

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.

Still, he and others agree, the new results indicate that stratospheric water vapor, especially in the lowest regions of the stratosphere, can have a significant impact on global average temperature trends when viewed in decade-long time frames.

Water vapor is the most abundant greenhouse gas in the atmosphere. By some estimates, it accounts for anywhere from 36 percent to 85 percent of the atmosphere's greenhouse effect, depending on whether clouds are included.

But the vast majority of that water vapor resides in the troposphere, which is the layer below the stratosphere. This is where the most of the day-to-day weather – and over long periods, climate – activity takes place.

Despite claims in some circles that global warming is over, the past decade was the warmest on record globally, according to records compiled by the GISS. Four of the 10 years were in a statistical tie for second place for the distinction of warmest year on record.

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