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May 2000
Science
Nan Rosenbloom and Tim Kittel. (Photos by Carlye Calvin.)

In some parts of the nation, what was once meadow is now forest, and what was once grassland is now cultivated. A CGD group is helping map these changes over the past century and their relationships to climate past, present, and future. The team recently found that U.S. vegetation may be storing considerably less carbon than researchers working on another recent study had speculated.

The Vegetation/Ecosystem Modeling and Analysis Project (VEMAP) includes several staff (pictured here) in CGD's Ecosystem Dynamics and the Atmosphere Section, led by Dave Schimel (now on leave at the Max Planck Institute for Biogeochemistry) and Tim Kittel. NCAR's VEMAP team has analyzed the last century's changes in climatology and land use (forest cover, agriculture, and the like) and put them onto a high-resolution grid. Working with projections of future climate from the more coarse global-scale models, VEMAP was used to estimate how trends in weather, climate, and land use might play out on the regional scale. These simulations are an important part of the U.S. National Assessment (see page 3).

A new study based on VEMAP data has found that land use, far more than atmospheric carbon dioxide levels or the vagaries of climate, influences how much carbon is stored by U.S. ecosystems. Dave is the lead author of an article on the study that appeared in the 17 March issue of Science.

Scientists have been searching for a carbon storage mechanism, or sink, to explain why atmospheric carbon dioxide levels are lower than expected as emissions rise. CO2 in the atmosphere fertilizes plants by stimulating photosynthesis, consequently increasing forest uptake of carbon. A shorter-term and more dramatic influence on carbon storage is climate and climate impacts: wildfires, volcanic eruptions, drought, and El Niño episodes can alter terrestrial carbon storage annually by as much as 100% in a given year. The authors examined the effects of both CO2 fertilization and climate events on U.S. carbon storage.

Cristina Kaufman and Steve Aulenbach.

For the period 1980-1993, the three models of terrestrial ecosystem biogeochemistry 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, had estimated net carbon uptake in the United States at levels of 1 to 2 billion tons, or 10 to 20 times that found in the VEMAP analysis.

The VEMAP 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 ecosystem models used by VEMAP 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 Dave. "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 says.

On the Web:
The VEMAP Project


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Edited by Bob Henson, bhenson@ucar.edu
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