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.

Regardless of the cause of the decline, the team’s modeling suggests that the decrease in water vapor concentrations 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 otherwise would have been, the researchers report. The team’s analyses using a climate model suggest that average global surface temperatures rose only 0.1 degrees Celsius during that period, rather than the 0.14 degree increase expected because of increases in other greenhouse gases.

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

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

Andrew Dessler, an atmospheric scientist at Texas A&M University in College Station says he thinks the team has identified a new source of short-term variability in climate, one different from long-term drivers such as anthropogenic greenhouse gases. And even though the effect seems to be substantial, the decrease in water vapor may be temporary.

What’s more, Dessler says, humans can’t depend on a continued decline in water vapor in the lower stratosphere to slow surface warming further in the long term. “Water vapor is scarce in the lower stratosphere already, and you can’t drop below zero,” he notes. “This is not going to save our bacon.”

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