Continents Growing Wetter as Globe Warms

By R. Monastersky at "Science News" (Vol. 152, P341. Nov. 29, 1997)

A study of global precipitation records going back to 1900 shows that the United States and other land areas in the midlatitudes have grown wetter, whereas the tropics have generally dried out, reports Aiguo Dai of the National Center for Atmospheric Research in Boulder, Colo. Such changes offer a taste of the kinds of precipitation shifts predicted to accompany greenhouse warming, the researchers say.

Rain and snow patterns profoundly influence social well-being by controlling factors ranging from food production to the frequency of floods. Concern about the potential for changes in precipitation underlie the international climate treaty negotiations taking place next week in Kyoto, Japan.

"Precipitation is what we really need to address. A lot of natural disasters are related more to precipitation than to temperature," says Inez Y. Fung of the University of Victoria, British Columbia. Dai, Fung, and Anthony D. Del Genio of NASA's Goddard Institute for Space Studies in New York report their finding in the November Journal of Climate.

Dai and his colleagues examined monthly precipitation records from 5,328 meteorological stations for the period 1900 through 1988. Using a statistical technique that teases out independent patterns in the data, they minimized errors introduced when instruments were changed or stations moved.

"This is by far the most comprehensive compilation of precipitation data. It is a much larger and cleaner data set than has existed before," says Fung.

On a global average, precipitation onto land areas increased through the first half of the century but has decreased since the mid-1970s, resulting in a small net gain. Much of the recent drop comes from a prolonged dry period in the Sahel region just south of the Sahara Desert.

The biggest increases have occurred in mid- and high-latitude areas, such as North America, norther Eurasia, Australia, and Argentina. Over the same period, the Philippines, Malaysia, and the Mediterranean lands have dried out.

This pattern matches the general picture emerging from computer climate models, say the researchers. In simulations of greenhouse warming, increased air temperatures cause more water to evaporate from Earth's surface. In the tropics, much of this additional moisture remains in the atmosphere because warmer air can more readily hold extra water vapor. In the middle and high latitudes, the vapor-storing capacity of the atmosphere also rises but not enough to hold most of the additional moisture, which drops as precipitation.

Despite the broad agreement between models and observations, Hery F. Diaz of the National Oceanic and Atmospheric Administration in Boulder cautions against concluding that green house gases have caused this century's precipitation changes.

In fact, he notes, early models called for greenhouse warming to increase the frequency of midcontinent droughts in North America, whereas U.S. data show an increase in precipitation.

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