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WASHINGTON, DC, June 22, 2001 (ENS) - An international consortium of scientists has issued a revised estimate of the U.S. role in the worldwide accumulation of carbon dioxide in the atmosphere, a major cause of global warming. The study, published in today's issue of the journal "Science," reconciles what had appeared to be conflicting measurements about the size of the U.S. "carbon sink" - an effect that drains carbon from the air and stores it in the land. The Princeton University led research group found that the continental U.S. is now absorbing one-third to two-thirds of a billion metric tons of carbon per year. The main reason is that U.S. trees and shrubs, which are recovering from past clearing, are drawing great volumes of carbon dioxide from the air and using the carbon to build massive tree trunks, branches and foliage. The suppression of natural forest fires also is causing an increase in vegetation. The study is the work of 23 scientists who held differing views about the size of the carbon sink. At the center of the dispute was the method of measuring the sink. One approach is to take samples from the atmosphere and estimate gains and losses of carbon dioxide as winds blow across the country. This strategy has yielded varying answers depending on the exact method used. Another approach is to inventory the places carbon can accumulate in the land - including trees, soils, landfills and reservoirs - and estimate how that inventory is changing over time. This land based approach gave very small estimates for the carbon sink, but none accounted for all the places carbon accumulates. Despite the large U.S. carbon sink, the nation still pumps a tremendous amount of carbon into the atmosphere. The burning of fossil fuels in the U.S. releases about 1.4 billion tons of carbon each year. Taking into account the carbon sink, 800 million to 1.1 billion tons accumulate in the atmosphere each year. The new analysis eliminates the possibility that the U.S carbon sink is big enough to equal the U.S. fossil fuel release, as some had speculated based on earlier studies. Princeton's Stephen Pacala, lead author of the new study, emphasized that the carbon sink should not be seen as offsetting the U.S. carbon emissions from fossil fuels. A large part of the sink is the result of the land taking back enormous quantities of carbon that were released due to heavy farming and logging in the 19th and early 20th centuries. "When we chopped down the forests, we released carbon trapped in the trees into the atmosphere. When we plowed up the prairies, we released carbon from the grasslands and soils into the atmosphere," said Pacala, who is a professor of ecology and evolutionary biology. "Now the ecosystem is taking some of that back." The sink will disappear over the next 50 to 100 years as U.S. ecosystems complete their recovery from past land use, Pacala said. "The carbon sinks are going to decrease at the same time as our fossil fuel emissions increase," Pacala said. "Thus the greenhouse problem is going to get worse faster than we expected."
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