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 land was either crops or forests and there were no nearby cities or water bodies, which can have their own distinct effects on temperatures. The cropland sites were predominantly in the Midwest, where 80% of the land is now under cultivation; the forested stations were in the Northeast, where just 20% of the land is now agricultural.
The data showed that the daily temperature range-- the difference between the highest (usually daytime) and lowest (usually nighttime) temperature in a day-- was lower in the Midwest than in the forested Northeast. This was because the daytime heating of agricultural stations across the Midwest was consistently lower than that of the forested northeastern stations.
The result was a surprise, because previous regional climate studies showed that the Midwest should have a larger daily temperature range than the Northeast, due to the moderating influence of clouds on daytime heating. The eastern United States is generally cloudier than the Midwest, and more clouds reflect more solar energy back into space. The daily high temperatures at the surface in the East should, therefore, rise less than in the unshielded Midwest. Bonan's new study found that temperatures in the Midwest did not rise as much during the day as they did in the Northeast, contrary to what was expected from these regional differences in cloud cover.
The seasonal character of the differences in temperatures between the agricultural Midwest and forested Northeast suggested a strong influence of the landscape. Bonan found that the cooling was most prominent in the Midwest in late spring and summer, just when crops reached their full growth. The temperature difference diminished in the fall after the time of harvest.
In 1850 croplands were on their way to being the dominant land cover in the Northeast, but forests and grasslands still dominated the Midwest, with only 5% of the land under cultivation. Just 30 years later, when northeastern croplands reached their highest level, the Midwest had caught up to match the Northeast with 50% of its land area cultivated. Cropland area steadily declined in the Northeast as forests returned; the spread of midwestern fields continued for the next 100 years, peaking at about 80% in the 1980s. Cropland in the Northeast is now less than 20% of the region.
To make sure that the results he was seeing were not happening in only one decade of the temperature/land-use measurements, Bonan also analyzed a 100-year record of U.S. temperatures. Before 1940, when the two regions had more similar amounts of cropland, the difference in regional daily highs was much smaller than it is today. Since 1940, as agriculture continued to spread across the Midwest and northeastern farm lands returned to forests, the temperature difference steadily increased. The Northeast became warmer in the spring and summer as forests returned.
Bonan is currently using a computerized model of the climate to further investigate the impact of historical deforestation on the eastern United States. This project is funded by NASA's Earth Observing System.
The paper, "Observational Evidence for Reduction of Daily Maximum Temperature by Croplands in the Midwest United States," appears in the June 1 issue of the American Meteorological Society's Journal of Climate.
NCAR is managed by the University Corporation for Atmospheric Research, a consortium of 66 universities offering Ph.D.s in atmospheric and related sciences.
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