U.S. carbon storage: Land use dominates carbon dioxide, climate

March 15, 2000

BOULDER--A new study has found that land use, far more than atmospheric carbon dioxide levels or the vagaries of climate, influences how much carbon is stored by ecosystems each year across the continental United States. Previous estimates of total U.S. carbon storage may have greatly overstated the actual levels. The results appear in the March 17 issue of the journal Science.

Lead author David Schimel is a senior scientist at the National Center for Atmospheric Research (NCAR) and head of the Max Planck Institute for Biogeochemistry in Jena, Germany. NCAR's primary sponsor is the National Science Foundation.

Scientists have been searching for a carbon storage mechanism, or sink, to explain why atmospheric carbon dioxide (CO2) levels are lower than expected as emissions rise. The carbon mystery is key to understanding 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 and more dramatic influence on carbon storage is climate: wildfires, volcanic eruptions, drought, and El Nino episodes can alter terrestrial carbon storage annually by as much as 100% in a given year. Schimel and his team examined the effects of both CO2 fertilization and climate events on U.S. carbon storage.

For the period 1980-1993, the three computer models used in the study agree within 25% that a U.S. land carbon sink resulting from CO2 fertilization and climate effects amounts to 0.1 billion tons per year--about a third of the total amount of stored carbon estimated from inventory data. Uptake of the other 0.2 billion tons, the authors conclude, is due to regrowth on abandoned agricultural land or where forests were harvested before 1980.

Last year a group of Princeton University researchers, focusing on the role of atmospheric CO2 fertilization, estimated net carbon uptake in the United States at levels of 1-2 billion tons, or 10 to 20 times that found in Schimel's analysis. The Princeton results indicate that carbon absorption on land is greater in the Northern Hemisphere and suggest that the United States plays a huge--and disproportionate--role in global carbon storage.

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

"To predict and plan for future climate change, we need to fully understand the amount of carbon being stored both in the U.S. and globally, and what controls that storage," says Schimel. "The next step is to quantify the North American carbon sink." A new and improved observing strategy--including airborne observations, remote sensing, surface flux measurements, and computer modeling--could resolve the discrepancies between the VEMAP and Princeton estimates of carbon storage, he said.
VEMAP is sponsored by the Electric Power Research Institute, the National Aeronautics and Space Administration, and the U.S. Forest Service. NCAR is managed by the University Corporation for Atmospheric Research, a consortium of more than 60 universities offering Ph.D.s in atmospheric and related sciences.

National Center for Atmospheric Research/University Corporation for Atmospheric Research

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