A new report suggests the slowdown in global warming in the last few years appears to have been caused by a decline in water vapor in the stratosphere.
While climate warming is continuing, the increase in temperatures was not as rapid as in the 1990s.
Balloon and satellite observations show the amount of water vapor in a layer about 10 miles high declined after 2000.
The researchers said the decline of water vapor in the stratosphere would reduce the rate of global warming expected from other gases such as carbon dioxide.
"Stratospheric water vapor concentrations decreased by about 10 percent after the year 2000," the researchers said in a study published in the peer-reviewed journal Science. "This
acted to slow the rate of increase in global surface temperature over 2000-2009 by about 25 percent compared to that which would have occurred due only to carbon dioxide and other greenhouse gases."
The reason for the decline is unknown, according to researchers led by Susan Solomon of the National Oceanic and Atmospheric Administration.
They added, however, that "more limited data suggest that stratospheric water vapor probably increased between 1980 and 2000, which would have enhanced the decadal rate of surface warming during the 1990s by about 30 percent compared to estimates neglecting this change."