Slowdown in Warming Linked to Water Vapor

By GAUTAM NAIK
Climatologists have puzzled over why global average temperatures have stayed roughly flat in the past decade, despite a long-term warming trend. New research suggests that lower levels of water vapor in the stratosphere may partly explain the anomaly.
The study, appearing in the journal Science, points out that the concentration of water vapor in the stratosphere has dropped about 10% in the past decade, triggered by unexplained cooler temperatures at certain high altitudes above the tropics. The study concludes that in the last decade the decline in water vapor slowed the rate of rising temperatures by about 25%, thus partly negating the heat-trapping effect of increasing greenhouse gases such as carbon dioxide and methane.
The recent fluctuation—the flattening of temperatures since the year 2000—isn't merely of academic interest. Those skeptical of man-made global warming say the temperature anomaly supports their case. Others say it is merely a blip, and that warming remains the long-term trend.
"There is slow warming that has taken place over the last 100 years," says Susan Solomon, lead author of the study and a researcher at the National Oceanic and Atmospheric Administration in Boulder, Colo. "But from one decade to another, there can be fluctuations in the warming trend," such as those caused by water vapor and other factors.
Separate findings suggest that fleeting changes in ocean currents and alterations in solar activity may partly explain the recent flat-temperature trend. The study in Science uses fresh and more-accurate satellite data to conclude that water vapor also likely contributed to the flattening of the global warming trend since 2000.
Not only is water vapor the planet's most abundant greenhouse gas, it also is known to amplify the warming effect of other such gases, including carbon dioxide. Scientists refer to the process as a positive feedback loop: Higher temperatures lead to higher concentrations of water vapor, which then absorbs more thermal energy radiated from the Earth, which further warms the atmosphere.
Water vapor is formed through evaporation from the Earth's bodies of water. A key factor that affects how much water vapor enters the stratosphere is the coldest temperature that air encounters as it rises from the Earth.
Most of this upward movement occurs in the tropics—a region where "cold point" temperatures have dropped in the past decade.
As a result of these lower temperatures, less water vapor ended up in the stratosphere. That, in turn, helped lower the warming rate, the study concludes.
The overall picture is still far from complete. Water vapor's role may be important, but "it doesn't rule out other contributing factors," such as changes in ocean currents and solar activity, says Dr. Solomon.
Nor do current warming models fully account for all the complexities of water-vapor shifts in the stratosphere. And scientists have yet to pin down why cold point temperatures in the tropics fell in the past decade.