Findings from two studies of smoke pollution from forest burning in the Amazon are detailed in the current issue of the journal Science. In the first paper, Meinrat O. Andreae of the Max Planck Institute for Chemistry in Mainz, Germany, and his colleagues report that in cases of heavy pollution, smoke suppresses rainfall, allowing the aerosols to penetrate the upper levels of the atmosphere. As a result, the clouds appear to smoke. Ultimately, the smoke aerosols can alter the amount of radiation reaching the earth and encourage long-distance transportation of the smoke. And when the aerosol- and water-laden clouds eventually release their precipitation, they generate intense thunderstorms and large hail instead of the usual moderate rainfall. "The invigorated storms release the latent heat higher in the atmosphere," the authors write. "This should substantially affect the regional and global circulation systems."
In the second study, a team led by Ilan Koren of the NASA Goddard Space Flight Center analyzed satellite data from the Amazon during the dry season and found that scattered cumulus-cloud coverage fell from 38 percent when the air was clean to zero in heavy smoke conditions. The incoming heat resulting from this reduction in cloud cover, they say, can swamp the cooling effects of the scattering of solar radiation by the smoke particles. This, the researchers conclude, may help explain "why Earth warmed substantially in the last century despite the expected aerosol cooling effect." --Kate Wong