Big Chill for the Greenhouse

By EUGENE LINDEN

At first Tim Barnett did not believe his own data. "I thought it was an error in the computer code," says the climate researcher at California's Scripps Institution of Oceanography. For one thing, the little-known phenomenon his model was predicting had not been witnessed since the mid- 1970s. By last summer, however, Barnett's forecast was borne out by a "monstrous" 7 degreesF plunge in ocean surface temperatures off * equatorial South America. The drop heralded the arrival of a mysterious weather pattern called La Nina, which brings unusually cold temperatures to the eastern Pacific. La Nina has since swept to the center of the climatic stage recently vacated by its better known heat-producing sibling El Nino.

Already La Nina has been credited with a role in causing this summer's drought in the Midwest, the deluges that flooded Bangladesh in September and the severe hurricane season in the Caribbean and the Gulf of Mexico. While widespread attention has been paid to the greenhouse effect -- the trend toward global warming due to the increase of carbon dioxide and other gases in the atmosphere -- some scientists believe that this winter La Nina will bring on a dramatic, though probably temporary, drop in average global temperatures. Says meteorologist and oceanographer James O'Brien of Florida State University: "We are predicting that by next year, average global temperature will retreat to 1950s levels, slowing up planetary warming by 30 to 35 years."

The name La Nina, Spanish for "the girl," was concocted two years ago to distinguish the "cold event" from El Nino, "the boy" or "Christ child," so named because it usually makes its appearance near Christmastime. Marked by an influx of warm water and winds from the western Pacific, El Nino is known to bring heavy winter rains to Peruvian deserts and warm weather to the U.S. West Coast. The two systems make up the extremes of a giant meteorological system called the Southern Oscillation that links the ocean and atmosphere in the Pacific. Normally the system functions as a giant heat pump, distributing energy from the equator to the higher latitudes through storms brewed over the warm western Pacific. In conjunction with the oceans, these climatic patterns affect much of the world's weather, ranging from the monsoon season in southeast Asia and the Indian subcontinent to rainfall in South America and Africa.

Essentially, La Nina exaggerates the normal conditions of the Southern Oscillation. Because its last occurrence predated sophisticated satellite data gathering, the phenomenon is not very well understood. Scientists do know, however, that during a La Nina, easterly trade winds are stronger, the waters of the eastern Pacific off South America are colder and ocean temperatures in the western equatorial Pacific are warmer than normal. The result: coastal deserts in Peru and Chile become even drier than normal, and the subcontinent is inundated by heavier-than-usual rainfall and, often, flooding.

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.