Ramanathan credits the clouds' formation to a combination of the area's tropical meteorology, outdated technology use, and rapid industrialization.
These dark particles, especially near urban and industrial regions, may add significantly to heating in the atmosphere, cautions Scripps' Craig Corrigan, a study co-author.
"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," Corrigan said.
In contrast, lighter-colored aerosols don't absorb solar energy the way darker particles do.
These nonabsorbant particles act like a parasol over Earth, reflecting energy back into space (read "Extreme Global Warming Fix Proposed: Fill the Skies With Sulfur" [August 4, 2006]).
"If you go into a parking lot, the white sidewalk is cooler than the black asphalt," Corrigan explained.
But unlike greenhouse gases, light and dark aerosols are not distributed uniformly throughout the globe, said Peter Pilewskie, an atmospheric scientist at the University of Colorado at Boulder who was not involved in the study.
Averaging the effects of aerosols worldwide masks regional processes that "we need to truly understand when we put all the pieces of the planet together," Pilewskie said.
"Lawn Mower With Wings"
To better understand the brown clouds, the researchers sent three unmanned aerial vehicles into the haze.
Each flyer was "the size of a small lawn mower with wings," Corrigan said.
The three vehicles simultaneously flew above, through, and below segments of the brown clouds lingering over the Maldives, an island country in the Indian Ocean.
During 18 missions in March 2006 the vehicles mapped the clouds' makeup and measured the solar energy they soaked up.
"It just so happened NASA launched this CALIPSO satellite, which gave us a precise measurement of the thickness of the cloud," Ramanathan said.
The combined data are detailed in tomorrow's issue of the journal Nature.
In 2008 NASA is planning to launch a satellite called Glory that will carry a new sensor to determine how much energy aerosols absorb from the sun.
"Our understanding of how air pollution and these brown clouds are influencing climate change is evolving," Ramanathan said.
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