Less Water Vapor Slows Earth’s Warming Trends, Researchers Say

Sindya N. Bhanoo, The New York Times 28 Jan 10;

A decrease in water vapor concentrations in parts of the middle atmosphere has contributed to a slowing of Earth’s warming, researchers are reporting. The finding, they said, offers part of the explanation for a string of years with relatively stable global surface temperatures.

Despite the decrease in water vapor, the study’s authors said, the overall trend is still toward a warming climate, primarily caused by a buildup in emissions of carbon dioxide and other heat-trapping gases from human sources.

“This doesn’t alter the fundamental conclusion that the world has warmed and that most of that warming has to do with greenhouse gas emissions caused by man," said Susan Solomon, a climate scientist at the National Oceanic and Atmospheric Administration and the lead author of the report, which appears in the Jan. 29 issue of the journal Science.

Water vapor, a potent heat-trapping gas, absorbs sunlight and re-emits heat into Earth’s atmosphere. Its concentrations in the stratosphere, the second of three layers in the atmosphere, appear to have decreased in the last 10 years, according to the study.

This has slowed the rate of Earth’s warming by about 25 percent, Dr. Solomon said.

“We use the 10-10-10 to describe it,” she said. “That is, a 10 percent change in water vapor, 10 miles above our head, over the past 10 years.”

The study also found that from 1980 to 2000, an increase in water vapor sped the rate of warming — the result of an increase in emissions of methane, another greenhouse gas, during the industrial period. Methane, when oxidized, produces water vapor. Why a decrease in water vapor has occurred in the last 10 years is still unknown.

Dr. Solomon emphasized that the study focused on the atmosphere’s middle layer, not to be confused with the troposphere, Earth’s first layer. It has been known for years that water vapor in the troposphere amplifies the effect of greenhouse gas emissions.

Some climate skeptics have claimed that a spate of years with relatively stable temperatures indicates that the threat of global warming has been overblown.

Last week, the National Aeronautics and Space Administration released figures indicating that the decade ending in 2009 was the warmest on record.