"Brown Clouds" Contribute to Himalaya Glacier Melt

Anne Casselman
for National Geographic News
August 1, 2007
Soot-filled "brown clouds" over the Indian subcontinent warm the lower atmosphere just as much as greenhouse gases do, a recent study reports.

Such clouds contain aerosols—tiny particles suspended in the air that are known to create a general cooling effect that could mitigate global warming.

But the latest study suggests that aerosols can be responsible for regional warming. Specifically, the clouds of aerosols over India enhance atmospheric warming there by 50 percent.

"We found this brown cloud can cover the entire North Indian Ocean, an area the size of the continental United States," said lead author Veerabhadran Ramanathan, a climate scientist at Scripps Institution of Oceanography at the University of California, San Diego.

The haze of brown clouds over the region can be up to two miles (three kilometers) thick, Ramanathan said.

And the haze touches the lower parts of the glaciers in the Himalaya mountain range, said study co-author David Winker, principal investigator of the CALIPSO satellite at NASA's Langley Research Center.

This suggests that the brown clouds may be contributing to glacial melting in the Himalaya.

(Related: "Mountain Glaciers Melting Faster Than Ever, Expert Says" [February 16, 2007].)

How Now Brown Cloud?

Brown clouds contain dark aerosols such as soot that are released into the atmosphere by burning organic matter.

These particles absorb solar energy and then release it to the surrounding air as heat.

Natural forces such as forest fires can create soot, but so can human actions such as burning fossil fuels.

Ramanathan credits the clouds' formation to a combination of the area's tropical meteorology, outdated technology use, and rapid industrialization.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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