Science: Big Chill for the Greenhouse

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The current La Nina, scientists believe, displayed just such an effect in Bangladesh, where heavy rains and flooding have killed more than 1,000 people in recent months. They also suspect the high-altitude easterly winds that accompany the system of helping to foster this year's severe hurricane season in the southern Atlantic. Kevin Trenberth, chief of climate analysis at the National Center for Atmospheric Research in Boulder, goes further, attributing this summer's drought in the Midwest in part to La Nina. One unusual characteristic of the present ocean cooling, he explains, is an accompanying warm patch of water near Hawaii. Trenberth thinks this conjunction pushed part of a tropical "convergence zone," where Pacific storms are formed, northward, thus forcing storms borne by the jet stream hundreds of miles north of their usual path over the U.S. Great Plains to Canada.

Climatologists are leery of predicting exactly how a vast atmospheric event like La Nina will affect weather in the U.S. or any other part of the world during the coming months. "The United States is a small area, and a subtle change in one area may have a profound effect on what we experience here," explains Chet Ropelewski of the National Oceanographic and Atmospheric Administration. But many scientists are confident that this La Nina will have a broad effect on global temperatures, mainly by virtue of cooling an estimated 10% to 15% of the oceans -- "a big piece of the world," as Florida State's O'Brien puts it. By contrast, the massive energy released in the two El Ninos during the past decade may have pushed average global temperatures upward, possibly leaving a false impression of a global warming trend.

La Nina may for a time offset the greenhouse effect by aiding the absorption of carbon dioxide before it accumulates in the atmosphere. According to atmospheric chemist Charles Keeling of Scripps, a La Nina may slow the increase in atmospheric CO2 by stimulating massive plant growth in areas of heavy rainfall. Keeling believes this La Nia might reduce CO2 in the atmosphere by roughly 40% of the total produced annually by worldwide fossil- fuel burning.

As yet, climatologists do not understand the mechanism that links the cycle between El Nino and La Nina. When they do, it may actually be possible to predict rainfall in the American Midwest by measuring snowfall in the ! Himalayas. "There is a framework into which all these pieces fit," says Scripps' Barnett. "If we could see how all these pieces work together, we could really do some long-range forecasting." In the meantime, La Nina seems set to cast a corrective, if temporary, chill on global warming.