Science News - February 27, 2010

10 percent

Drop in water vapor levels in part of lower stratosphere, 2000–01

25 percent

Reduction in expected rise in global temperature linked to water vapor loss, 2000–09

Water vapor drop dampens temps

Global warming slowed by decline in upper atmosphere’s H2O

By Sid Perkins

A sudden and unexplained drop in the amount of water vapor high in the atmosphere almost a decade ago has substantially slowed the 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 about 10 percent and have remained relatively stable since. 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 the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory in Boulder, Colo.

Because water vapor is a potent greenhouse gas, the decline has slowed the increase of global temperatures, Rosenlof, her NOAA Boulder colleague Susan Solomon and others report online January 28 in Science.

“This is such a sudden decrease, we can’t explain what’s behind it,” says Rosenlof. One large source of water vapor in the stratosphere is the oxidation of methane, she notes. But the decline 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 levels of stratospheric water vapor, she says.

Regardless of the cause of the decline, the team’s modeling suggests that the decrease in water vapor in the lower stratosphere has slowed down average global warming. The rate of increase in the average global surface temperature from 2000 to 2009 was about

25 percent lower than it other wise would have been, the researchers report. The analyses suggest that average global surface temperatures rose only 0.1 degrees Celsius during that period, rather than the 0.14-degree rise expected because of increasing concentrations of other greenhouse gases.

The researchers speculate that the amount of water vapor gradually rising into the stratosphere at tropical latitudes has decreased, possibly because of a shift in global patterns of sea-surface temperatures that influence rates of evaporation and water vapor movement.

The new findings “are a nice dem-
onstration of the sensitivity of the cli-
mate to water vapor concentrations in
the lower stratosphere,” says Andrew
Gettelman, an atmospheric scientist at
the National Center for Atmospheric
Research, also in Boulder.