Although previous research had seemed to indicate that aerosols could create a general cooling effect in the atmosphere — thus helping mitigate the effect of global warming — a new study has revealed that they may in fact warm it just as much as greenhouse gases. In fact, these "brown clouds" — soot-filled aerosol clouds — have been increasing atmospheric warming over India by about 50%.
According to the study's lead author, Veerabhadran Ramanathan of the Scripps Institution of Oceanography in San Diego, the brown clouds could cover the North Indian Ocean, an area close to that of the continental U.S., and their haze could be up to 2 miles (3 km) thick. Because the haze reaches the lower parts of the Himalaya mountain range, Ramanathan and his colleagues believe the clouds could play an important role in the recently observed glacial melting.The brown clouds contribute to atmospheric warming by absorbing the sun's energy and then releasing it to the surrounding air as heat. "When we introduce a little more of our own pollution—especially when it's dark black soot—there's a more dramatic effect on warming," said Craig Corrigan, a climate scientist from Scripps and a co-author on the study. Indeed, he draws a distinction between the darker-colored aerosols — such as those found in the brown clouds — and lighter-colored ones which don't absorb solar energy. These, in contrast, help reflect energy back into space and thus spur a cooling effect in the atmosphere.
To assess the effect of the brown clouds, Ramanathan and his co-authors sent 3 unmanned aerial vehicles into the haze to map their makeup and measure the amount of solar energy they absorbed. The results they obtained after conducting 18 missions in March 2006 confirmed their initial hypothesis.
NASA plans on launching a new satellite — dubbed "Glory" — in 2008 that will carry a special sensor to determine the exact quantity of solar energy absorbed by the aerosols. "Our understanding of how air pollution and these brown clouds are influencing climate change is evolving," said Ramanathan.