20 February 2002

Researcher Finds Biomass Burning Increases Stratospheric Moisture

Contributes to milder winters in northern hemisphere

Tropical biomass burning, used to clear forests or grassland for agricultural purposes, has helped double the moisture content in the Earth's stratosphere over the last 50 years, a Yale University researcher has concluded after examining satellite weather data.

Steven Sherwood, assistant professor of geology and geophysics at the Connecticut-based institution, said in a February 20 press release that the increased humidity -- as well as carbon dioxide from the burning of fossil fuels -- has caused a cooling trend in the stratosphere that is now contributing to milder winters in North America and Europe. By contrast, harsher winters have resulted in the Arctic.

Sherwood said in an interview that it is now believed by researchers that the cooling of the Earth's stratosphere should change the atmospheric circulation in such a way as to reduce the number of "cold air outbreaks" -- where cold polar air invades Europe or North America during the winter. This would lead to milder winters at these high latitudes, but more severe winters in Polar Regions since the cold air would remain trapped there instead.

This effect is separate from the well-known overall warming effect on the Earth's surface due to greenhouse gases, although the same gases, including water vapor, causes both effects.

Sherwood, whose article appears in the current issue of the journal Science, said that higher humidity in the stratosphere also helps catalyze the destruction of the Earth's ozone layer. The stratospheric ozone layer helps protect humans and animals from the harsh rays of the sun.

In the study, funded by the National Aeronautics and Space Administration (NASA), Sherwood examined a combination of data from a NASA satellite launched in the 1990s and operational weather satellite data archived at the Goddard Institute of Space Science in New York.

Following is the text of the press release:

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YALE UNIVERSITY

February 20, 2002

Increased Water Vapor in Stratosphere Possibly Caused by Tropical Biomass Burning

New Haven, Conn. -- The doubling of the moisture content in the stratosphere over the last 50 years was caused, at least in part, by tropical biomass burning, a Yale researcher has concluded from examining satellite weather data.

Tropical biomass burning is any burning of plant material. In the tropics this is usually the clearing of forest or grassland for agricultural purposes, mostly before the growing season.

"In the stratosphere, there has been a cooling trend that is now believed to be contributing to milder winters in parts of the northern hemisphere; the cooling is caused as much by the increased humidity as by carbon dioxide," said Steven Sherwood, assistant professor of geology and geophysics whose article appears in this week's issue of the journal, Science. "Higher humidity also helps catalyze the destruction of the ozone layer."

Cooling in the stratosphere causes changes to the jet stream that produce milder winters in North America and Europe. By contrast, harsher winters result in the Arctic.

Sherwood said that about half of the increased humidity in the stratosphere has been attributed to methane oxidation. It was not known, however, what caused the remaining added moisture.

In a study funded by the National Aeronautics and Space Administration, Sherwood examined a combination of data from a NASA satellite launched in the 1990s and operational weather satellite data archived at the Goddard Institute for Space Science in New York.

In particular, he studied monthly and yearly fluctuations of humidity in the stratosphere, relative humidity near the tropical tropopause, which is the place where air enters the stratosphere, ice crystal size in towering cumulus clouds, and aerosols associated with tropical biomass burning.

"More aerosols lead to smaller ice crystals and more water vapor entering the stratosphere," Sherwood said. "Aerosols are smoke from burning. They fluctuate seasonally and geographically. Over decades there have been increases linked to population growth."

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