Why is the southeastern U.S. getting cooler while the rest of the globe is warming? Thank the trees, say some researchers.

On sweltering summer days, trees and other plants emit volatile organic compounds, such as isoprene, which combine with manmade soot and other aerosols in the atmosphere to produce a cooling haze, says environmental scientist Allen Goldstein of the University of California, Berkeley.

Goldstein and his colleagues used satellite and ground sensor data to track air pollution. Over time, the cooling induced by the atmospheric haze has outpaced the warming due to rising carbon dioxide levels in the atmosphere, according to findings published in Proceedings of the National Academy of Sciences. "Nobody realized until now that enough of these aerosols were forming to influence cooling over an entire region," Goldstein says.

Of course, it would require a constant spew of aerosols to mimic the effect elsewhere (or on a global scale), enough to turn the sky gray instead of blue and make it difficult to breathe. So plant-produced haze isn’t likely to become a geoengineering solution anytime soon.

But the results go a long way to explain why high temperatures in the southeastern U.S. are dipping even as global CO2 levels rise. It isn't that there are more trees per se, it's that scientists now have better ways of measuring pollutants and their effect on local temperatures.

Image: © iStockphoto.com / Sebastien Windel