Scientists reconcile opposing views of U.S. role in greenhouse gas problem

June 21, 2001

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 the June 22 issue of Science, reconciles what had appeared to be sharply 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 United States is currently 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 initially held strongly 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 directly from the atmosphere and estimate gains and losses of carbon dioxide as winds blow across the country. This strategy has yielded widely varying answers depending on the exact method used. Another approach is to inventory the myriad 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.

In 1998, a Princeton-led group published a paper using atmospheric data to estimate that southern Canada, the 48 states and Mexico collectively absorb 1.4 billion tons of carbon per year. That conclusion triggered strong criticism from scientists who believed it was an inflated figure.

The new study reconciles previous differences by including 27 different atmospheric methods and by performing an exhaustive land-based analysis. The scientists also took special care to base their calculations on the same time period and the same geographic region, focusing exclusively on the 48 states in the years 1980 to 1989. Through this process, the researchers found that the atmosphere- and land-based assessments actually agree with each other, within the margins of uncertainty of each method. The final answer of one-third to two-thirds of a billion tons per year is lower than the controversial 1998 result, but higher than all earlier land-based estimates and some previous atmospheric estimates.

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 United States 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 annually. 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 following the 1998 study.

Princeton's Stephen Pacala, the 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, and should not be viewed as a license to release more carbon. A large part of the sink is merely 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."

The problem is similar worldwide. Globally, the use of fossil fuels pumps 6.3 billion tons of carbon into the atmosphere each year. Another 1.6 billion tons are estimated to be released due to the widespread loss of forestland in some parts of the world. However, the oceans and land-based carbon sinks, such as the North American sink, absorb a large amount of carbon, leaving just over three billion tons to accumulate in the atmosphere each year. As U.S. and other carbon sinks mature and disappear, that rate of accumulation will increase dramatically.

There is some interplay between the U.S. carbon sink and emissions in other parts of the world. The study found that the United States absorbs 370 to 710 million tons of carbon each year, but 70 to 130 million tons of this are exported to other parts of the globe in the form of grain and other agricultural products and are released back into the atmosphere elsewhere when those products are used. Thus the total impact of the U.S. sink on worldwide carbon absorption is the removal of 300 to 580 million tons of carbon per year.
In addition to Princeton scientists, the Science paper includes authors from the University of New Hampshire, the French national research lab CNRS, the Woods Hole Research Center, the U.S. Forest Service, the U.S. Geological Survey, the Johann Wolfgang Goethe-Universitaet in Frankfurt, Germany, the Max Planck Institute for Biogeochemistry in Jena, Germany, Oregon State University and the Carnegie Institution of Washington in Stanford, Calif.

Princeton University

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