Shift from forest to crops lowers te...( land was either crops or forestsand t...)

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 thei
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Contact: Anatta
anatta@ucar.edu
303-497-8604
National Center for Atmospheric Research/University Corporation for Atmospheric Research
31-May-2001

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