Hot conversion

Use of forestland for agriculture causes climate change

Published: Sunday 15 July 2001

Change in land use from forest the large-scale conversion of forests to croplands in the midwestern usa over the last 100 years has led to a measurable cooling of the region's climate, shows the study conducted by Gordon Bonan, researcher at the National Centre for Atmospheric Research ( ncar ), in the us .

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.

"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. Crops, as compared to forests, reflect more sunlight, this increases cooling on the land. The impact of land use changes on climate is one of the hottest debates in the climate change discussions. Most of the work to date on the subject has been done using computer models and has focused on deforestation in the tropics, in areas such as the Amazon. This study is one of the first observational studies on the effect of temperate forest changes on regional climate. Bonan's earlier work has hinted at this cooling effect in the midwest us.

Since accurate temperature and land-use records do not exist for the us midwest 150 years ago, when agriculture began to deforest the region, Bonan relied on a direct 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 temperature records for 1986 to 1995, compiled from 65 us weather stations locatd near land that was either crops or forests and there were no nearby cities or water bodies, which could have had their own distinct effects on temperatures.

The cropland sites were predominantly in the midwest, where 80 per cent of the land is now under cultivation; the forested stations were in the northeast, where just 20 per cent of the land is now under agriculture.

The data showed that the daily temperature range -- the difference between the highest and lowest temperature recorded 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 climate studies had shown 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 parts of the us are 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 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. Cooling was most prominent in the midwest in late spring and summer, when crops reach 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 five per cent 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 per cent 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 per cent in the 1980s. Cropland in the Northeast us is now less than 20 per cent 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 us 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 computerised model of the climate to further investigate the impact of historical deforestation on the eastern us .

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