From: Sid Perkins, Science News
Published January 29, 2010 07:03 AM

Global Warming Slowed by Decline in Atmospheric Water Vapor

A sudden and unexplained drop in the amount of water vapor present high in the atmosphere almost a decade ago has substantially slowed the rate of warming at Earth's surface in recent years, scientists say.

In late 2000 and early 2001, concentrations of water vapor in a narrow slice of the lower stratosphere dropped by 0.5 parts per million, or about 10 percent, and have remained relatively stable since then. Because the decline was noted by several types of instruments, including some on satellites and others lofted on balloons, the sharp decrease is presumed to be real, says Karen Rosenlof, a meteorologist at NOAA's Earth System Research Laboratory in Boulder, Colo. 


And because water vapor is a powerful greenhouse gas, the decline has slowed the increase of global temperatures, Rosenlof, Susan Solomon, also of NOAA in Boulder, and their colleagues report online January 28 and in an upcoming Science.

"This is such a sudden decrease, we can't explain what's behind it," Rosenlof says. One large source of water vapor in the stratosphere is the oxidation of methane, she notes. But the decline in concentration of that gas detected by the researchers seems to be limited to a layer 2 kilometers thick in the lower stratosphere, while methane is found throughout the stratosphere. And even though scientists have discerned a leveling off in atmospheric methane in recent years, that trend doesn’t seem to be directly linked to the drop in the concentrations of stratospheric water vapor, she says.

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