The researchers used their results to help answer a major question that has been a subject of much contention: How big is the entire "carbon sink" of the continental United States?
According to their findings, the scientists estimate that U.S. forests and other terrestrial components absorb from one-third to two-thirds of a billion tons of carbon each year.
At the same time, reliable figures indicate that the United States emits more than two to four times that amount of carbon each year, about 1.4 billion tons.
Taking into account the carbon sink effect, 800 million to 1.1 billion tons of carbon accumulates annually in the atmosphere, the researchers say. This refutes the idea that the U.S carbon sink is big enough to equal the amount of carbon that U.S. factories emit through the burning of fossil fuels, as some studies have concluded.
The results of the Princeton-led study are particularly interesting because the 23 scientists who participated in the research and agreed on the conclusions initially held strongly differing views about the size of the U.S. carbon sink.
Pacala and his colleagues say the main reason the United States is drawing in a large volume of carbon is because many forests and areas of land that were logged or converted to agriculture in the last 100 years are now recovering with the growth of new vegetation.
These trees and shrubs absorb carbon dioxide from the air and channel it into the growth of massive tree trunks, branches, and foliage. This, in turn, gradually expands the overall size of the U.S. carbon sink.
Pacala emphasizes, however, that the U.S. absorption of carbon does not fully offset the emissions of carbon from fossil fuels and should not be seen as a license to release more carbon. A large part of the current sink effect, he said, is the land re-absorbing large quantities of carbon that were released during heavy farming and logging of the past.
"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. "Now the ecosystem is taking some of that back." But, he added, the sink effect will steadily decrease and eventually disappearas U.S. ecosystems complete their recovery from past land use.
"The carbon sinks are going to decrease at the same time as our fossil fuel emissions increase," he said. "Thus, the greenhouse problem is going to get worse faster than we expected."
Carbon Sink in China
In a separate study in Science, researchers reported on a similar carbon sink effect in China, which they attribute to the regrowth of logged forests and intensive planting of new forests.
Jingyun Fang, an ecology professor at Peking University in Beijing, and his colleagues noted that Chinese forests were heavily exploited from 1949 to the end of the 1970s. Since then, however, the government has undertaken wide-scale forest planting and reforestation, mainly to combat erosion, flooding, desertification, and loss of biodiversity.
An unintended consequence of this increase in vegetation was the growth of a carbon sink that is estimated to be on par with that of North American forests.
(c) 2001 National Geographic Society
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