In one of the oddities of parsing the regional effects of global climate change, researchers have noted that the U.S. Southeast has seen average high temperatures dip by as much as 0.38 degrees Fahrenheit (0.21 degrees Celsius) over the course of the 20th century. Curious, atmospheric scientist Robert Portmann of the National Oceanic and Atmospheric Administration (NOAA) and his colleagues decided to take a look at what might be causing it.

Their findings, published yesterday in the Proceedings of the National Academy of Sciences USA: that increasing spring rainfall during May and June each year in parts of the southeastern U.S. appeared to be keeping down average high temperatures, based on a study of thousands of readings from weather stations in the Global Historical Climatology Network Daily system.

"Wetter areas tend to have higher humidity and cloudiness and all of these could be causing reduced temperature trends in the Southeast U.S. compared with dryer areas," Portmann says. "However, one would not expect these long-term average quantities to produce negative trends in temperatures."

Instead, the researchers propose that urbanization and population growth in the area could be contributing more soot and other aerosols to the atmosphere, holding down temperatures. But similar areas in, for example, China show no such effect. So "something more uniquely tied to this region appears to be necessary," the researchers write, and offer the suggestion that the regrowth of forests in the region after clearing for agriculture in the 18th and 19th centuries is to blame.

After all, the Southeast puts into the atmosphere as much of the volatile organic compound isoprene—emitted by trees—as the Amazon, according to a 1999 study in Atmospheric Environment. That effect stops at the 100th meridian, which divides wetter eastern Texas from drier western Texas—as does the cooling effect.

"Dry places in the Southwest are experiencing hotter temperatures… maximum temperatures in the Southeast have actually decreased on average, and we show that this seems to be closely linked to the way rainfall changes moving east across the famous 'dry line' across the middle of the country," says NOAA atmospheric scientist and co-author of the report Susan Solomon. "The pattern of warming across the U.S. has a remarkable link to rainfall, especially for the maximum temperatures."

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