BOULDER--The large-scale conversion of forests to croplands in the midwestern United States over the last century has led to a measurable cooling of the region's climate, according to Gordon Bonan of the National Center for Atmospheric Research (NCAR). The new study, which appears in the June issue of the Journal of Climate, is the first to document the link between regional climate change and a major change in temperate forest cover. NCAR's primary sponsor is the National Science Foundation.
"Human uses of land, especially clearing of forest for agriculture and reforestation of abandoned farmland, are an important cause of regional climate change," concludes Bonan. The cooling is the result of the changeover of the region to crops, which reflect more sunlight back into space than forests.
The impact of land-use changes on climate is currently one of the most uncertain factors contributing to climate warming, according to the latest report from the United Nations/World Meteorological Organization Intergovernmental Panel on Climate Change, issued earlier this year. Most of the work to date on this subject has been with computer models and has focused on deforestation in the tropics in areas such as the Amazon. Bonan's is one of the first observational studies on the effect of temperate forest changes on regional climate.
His own earlier model results hinted at this cooling effect in the U.S. Midwest, but Bonan wanted to go further. "I wanted to see if this pattern really exists in nature."
Since accurate temperature and land-use records do not exist for the U.S. Midwest 150 years ago, when agriculture began to deforest the region, Bonan relied on a direct modern-day comparison between temperatures in predominantly forested areas and those in cropland areas to see if the different types of land cover were associated with different temperatures.
He used temperatures from 65 U.S. weather reporting stations from
1986 to 1995, where the surrounding
National Center for Atmospheric Research/University Corporation for Atmospheric Research