U.S. Carbon Storage: Land Use Dominates Carbon Dioxide, Climate

Lead author David Schimel is a senior scientist at the NationalCenter for Atmospheric Research (NCAR) and head of the Max PlanckInstitute for Biogeochemistry in Jena, Germany. NCAR's primarysponsor is the National Science Foundation.

Scientists have been searching for a carbon storage mechanism, orsink, to explain why atmospheric carbon dioxide (CO2) levels arelower than expected as emissions rise. The carbon mystery is key tounderstanding the impact humans are having on the earth's climate.

Atmospheric CO2 fertilizes plants by stimulating photosynthesis,consequently increasing forest uptake of carbon. A shorter-term andmore dramatic influence on carbon storage is climate: wildfires,volcanic eruptions, drought, and El Nino episodes can alterterrestrial carbon storage annually by as much as 100% in a givenyear. Schimel and his team examined the effects of both CO2fertilization and climate events on U.S. carbon storage.

For the period 1980-1993, the three computer models used in the studyagree within 25% that a U.S. land carbon sink resulting from CO2fertilization and climate effects amounts to 0.1 billion tons peryear--about a third of the total amount of stored carbon estimatedfrom inventory data. Uptake of the other 0.2 billion tons, theauthors conclude, is due to regrowth on abandoned agricultural landor where forests were harvested before 1980.

Last year a group of Princeton University researchers, focusing onthe role of atmospheric CO2 fertilization, estimated net carbonuptake in the United States at levels of 1-2 billion tons, or 10 to20 times that found in Schimel's analysis. The Princeton resultsindicate that carbon absorption on land is greater in the NorthernHemisphere and suggest that the United States plays a huge--anddisproportionate--role in global carbon storage.

Schimel's study is part of the Vegetation Ecosystem Modeling andAnalysis Project (VEMAP). His team used new, detailed historicalinformation on climate and an ensemble of three computer models tostudy carbon storage in the 48 states from 1895 to 1993. All threemodels simulate carbon storage in soil and vegetation within naturalecosystems, and one also simulates carbon in agricultural ecosystems.

"To predict and plan for future climate change, we need to fullyunderstand the amount of carbon being stored both in the U.S. andglobally, and what controls that storage," says Schimel. "The nextstep is to quantify the North American carbon sink." A new andimproved observing strategy--including airborne observations, remotesensing, surface flux measurements, and computer modeling--couldresolve the discrepancies between the VEMAP and Princeton estimatesof carbon storage, he says.

VEMAP is sponsored by the Electric Power Research Institute, theNational Aeronautics and Space Administration, and the U.S. ForestService. NCAR is managed by the University Corporation forAtmospheric Research, a consortium of more than 60 universitiesoffering Ph.D.s in atmospheric and related sciences.