Mysterious Water Vapor Drop in Stratosphere Slows Global Warming
by David Bielo, SCIENTIFIC AMERICAN, January 29,2010
Earth’s stratosphere is a cold, dry place, above the troposphere—the bottom layer of the atmosphere we breathe on a daily basis. Ruled by winds and hosting everything from bacteria to long-distance jet travel, about the only way that water gets into this high-altitude layer 10 kilometers above the Earth’s surface is when it billows up from the humid tropics, rising from the troposphere via the atmospheric interface known as the tropopause. But since 2001 there has been less water vapor in a narrow, lower band of the stratosphere thanks to cooler temperatures in the tropopause, and that may just be holding back global warming at ground level, according to new research published online in Science on January 28.
"We found that there was a surface temperature impact due to changes in water vapor in a fairly narrow region of the stratosphere," explains research meteorologist Karen Rosenlof of the National Oceanic and Atmospheric Administration’s (NOAA) Aeronomy Laboratory, one of the authors of the study. "The reason for the water vapor change is the temperature drop at the interface between the troposphere and the stratosphere over the tropics. What we don’t know is why the temperature dropped."
That temperature does seem to correlate, however, to sea-surface temperatures in the Pacific that, of course, follow El Niño–La Niña cycles, along with other trends. A new El Niño cycle—warmer surface waters—began last summer, which may mean that stratospheric water levels could change again. So this effect could either be the result of natural variability in Earth’s climate, or yet another effect of carbon dioxide and other greenhouse gases like water vapor trapping more heat and thus warming sea-surface temperatures. [Read rest of story]