Summer 2000

U.S. carbon storage is predominantly determined by land use

by UCAR Communications staff

The Vegetation/Ecosystem Modeling and Analysis Project (VEMAP) includes several members of NCAR's Climate and Global Dynamics Division under the leadership of NCAR scientists David Schimel (on leave at the Max Planck Institute for Biogeochemistry in Jena, Germany) and Timothy Kittell. The VEMAP team has analyzed the last century's changes in climatology and land use and put them on a high-resolution grid. Working with projections of climate from the more coarse global-scale models, the VEMAP data were used to estimate how trends in weather, climate, and land use might play out on the regional scale.

A new study based on VEMAP data has found that land use affects the amount of carbon stored in U.S. ecosystems far more than do atmospheric carbon dioxide levels and climatic factors. The results were reported in the March 17 issue of Science.

For years, scientists have been trying to pinpoint a carbon storage mechanism, or sink, that would explain why atmospheric carbon dioxide levels are lower than expected from the increasing rate of CO₂ emissions. Some evidence suggests that this "missing sink" is located in the Northern Hemisphere. The VEMAP group focused on three of the possible causes: carbon fertilization, climate, and land use. Atmospheric CO₂ fertilizes plants by stimulating photosynthesis, consequently increasing forest uptake of carbon. Climate has a shorter-term but more dramatic influence on carbon storage: drought, El Niño episodes, and other climatic effects can alter terrestrial carbon storage annually by as much as 100% in a given year. Changing land use affects carbon storage because different ecosystems hold different amounts of carbon—forests store much more than agricultural lands, for example.

The studies used three computer models, data on climate and atmospheric CO₂, and a host of information on land use: forest types, soil types, fertilization, planting and harvest dates, etc. For the period 1980–1993, the models agreed within 25% that a U.S. land carbon sink resulting from CO₂ fertilization and climate effects amounts to 0.1 billion tons per year, or 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 and in forests that were harvested before 1980.

Last year, a group of Princeton University researchers, focusing on the role of atmospheric CO₂ fertilization, estimated that the net carbon uptake in the United States was 1–2 billion tons, or 10 to 20 times the level found in the VEMAP analysis. According to the Princeton study, carbon absorption on land is greater in the Northern Hemisphere than elsewhere around the earth. The results also suggest that the United States plays a huge, and disproportionate, role in global carbon storage.

"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," said 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.