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Spring 2000

Agriculture keeps the Midwest cooler

by Carol Rasmussen

There were no scientific studies or international protests as 19th-century farmers cut down the dense forests of the upper Midwest--Wisconsin, Minnesota, Illinois, Ohio--to make way for corn and soybean fields. Nonetheless, like today's tropical deforestation, that change made its mark on climate. NCAR scientist Gordon Bonan (Climate and Global Dynamics Division) has confirmed that the switch from forest to cropland has cooled summer and fall temperatures in the region by 2-3°C (4-5°F).

Bonan had seen the effects of the deforestation in his earlier work using the NCAR Land-Surface and Community Climate Models. In these studies, he compared a simulation of the current agricultural land cover to one with the native forests. The change to cropland reduced daily maximum temperatures in summer and autumn, when crops are growing, while little affecting daily minimums; in other words, the diurnal temperature range decreased.

After publishing these results, "I decided to look into [the deforestation effect] in terms of observational evidence," Bonan said. Although good temperature records extend back to the forest days, there are few records of the pattern of land-use change. To get around that problem, Bonan explained, "I had to relate current geographic patterns of temperature to current geographic patterns of land use." He compared daily maximum and minimum temperatures over the past decade in the agricultural Midwest with those in New England, which is primarily forested. After accounting for the differences in climate between the two regions, he found that the diurnal temperatures were consistent with the model predictions.

The main cause for the cooling is the difference in albedo between crops and forest. "Trees tend to be pretty dark, so they absorb a lot of radiation," Bonan said. "Satellite observations show that the Corn Belt region has a higher albedo than nearby forested areas." Another contributor is the higher evaporation from croplands than from forests.

An earlier, unrelated study by Roger Pielke, Sr., of Colorado State University showed that agriculture also keeps the Colorado plains cooler. In that case, however, the main culprit was the introduction of irrigation in a semiarid climate, not the change in land cover. Since farmland in the upper Midwest is generally not irrigated, "my study was just crops vs. trees," said Bonan.

"As people try to reconstruct climate change over the past 100 years, they talk about increasing carbon dioxide, volcanic emissions, and even sulfate aerosols," Bonan noted. When it comes to land-use change, however, they rarely look beyond urbanization and today's tropical deforestation. "What this study is saying is that there's another human influence in how we use land that has to be accounted for."

Bonan's observational study is currently in review. His modeling studies appear in Climatic Change and Ecological Applications.


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Edited by Carol Rasmussen, carolr@ucar.edu
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Last revised: Thu May 4 14:53:14 MDT 2000