El Niño May Take Wind Out of Global Warming Sails

El Niño, the periodic warming of the Eastern Pacific, took the rap for two nasty weather events last month: the hurricane that swept Acapulco and the blizzard that dumped up to a meter of snow over the U.S. heartland. But El Niño also has an upside that may help researchers better understand global climate change. Findings in the current issue of Science suggest that, by warming global climate, an El Niño or any other warm period may help temporarily brake the ongoing rise in atmospheric carbon dioxide due to human activity.

Atmospheric carbon dioxide concentrations have increased more or less steadily over the past 20 years, continuing a trend more than a century old that is attributed largely to rising consumption of fossil fuels and large-scale destruction of forests by slash-and-burn agriculture and logging. Earth scientist Rob Braswell of the University of New Hampshire, Durham, and his colleagues analyzed shorter term fluctuations in carbon dioxide levels and--using powerful satellite-based techniques--global temperatures and plant growth after unusual warm spells, some of which are attributable to El Niño events. "We really didn't know what was going to happen, and we weren't confident we'd see anything conclusive," says Braswell.

But to their surprise, the team found that the rate of increase of atmospheric carbon dioxide levels slowed significantly about 2 years after each of four warm spells that occurred between 1980 and 1991, including the major El Niño of 1982-83. Global vegetation growth--as measured by light reflected from photosynthetically active leaves--also sped up after a comparable time lag, suggesting that the plants were sopping up the excess carbon dioxide. "It's a surprise to see such a clear delay given all the variables in global climate and plant growth," Braswell says.

The findings implicate ecosystem processes--perhaps interactions between soil microbes and plants--as a middleman between warming and plant growth. "These results are a major step forward in providing evidence for mechanisms that explain terrestrial responses to climate change," says ecologist Stuart Chapin at the University of California, Berkeley. Experts say it's unclear, however, whether such plant growth might restrain carbon dioxide buildup over the long haul.

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