The reforestation of former farmland over the last century has played an important role in reducing the accumulation of greenhouse gasses in the atmosphere, according to Princeton scientists.

The scientists, publishing in the Nov. 10 issue of Science, reported that changes in land use have been critical in allowing North American forests to regrow and soak up large amounts of carbon dioxide. Previous studies had suggested that other factors, such as the fertilizing effects of carbon dioxide, were spurring forests to absorb more carbon dioxide.

"Changes in the way we manage our land have had a real impact on the global environment," said the paper's lead author, John Caspersen.

The finding makes it clear, however, that this benefit will not continue indefinitely, because the regrowth of forests will slow as they mature. The results could have important implications for policymakers wrestling with the question of how to reduce the accumulation of carbon dioxide in the atmosphere.

Scientists have been trying for more than a decade to track the fate of carbon dioxide pumped into the atmosphere as a result of burning fossil fuels. Early studies showed that despite six billion tons of the gas emitted each year, only three or four billion tons accumulate in the atmosphere. Landmark studies from Princeton and elsewhere showed that trees and other land plants, which absorb carbon dioxide during photosynthesis, were taking up a large part of the "missing" carbon. Then, in 1996, a Princeton-led group reported that much of this absorption was happening in the United States and neighboring countries - a phenomenon called the "North American carbon sink."

Still, it was not clear what was causing North America to absorb so much carbon. Some evidence suggested that carbon dioxide itself would stimulate plant growth, thus causing more carbon dioxide uptake. Increased nitrogen pollution and global warming also could stimulate plant growth. Studies publis
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Contact: Steven Schultz
sschultz@princeton.edu
609-258-5729
Princeton University
9-Nov-2000