The soot and other pollution that make up “brown clouds” in skies over Asia warm the atmosphere as much as greenhouse gases, according to a team led by Scripps Institution of Oceanography at UC San Diego atmospheric chemistry professor V. Ramanathan.
That warming is jeopardizing the water supply of billions of people who live in that region by accelerating the melting of Himalayan glaciers, but there is hope that south Asian countries can begin to mitigate the problem now that it has been identified, said Ramanathan.
“Many climate scientists have concluded that the large-scale retreat of the Himalayan glaciers is due to global warming from greenhouse gases,” Ramanathan said. “Our study shows there is yet another gorilla in the form of atmospheric brown clouds which are contributing as much to the warming of the Himalayas.”
Ramanathan’s group described its findings in the Aug. 2 edition of the journal Nature that atmospheric brown clouds enhance solar heating of the lower atmosphere by about 50 percent. The combined heating effect of greenhouse gases and the brown clouds, which contain black carbon, trace metals, and other particles from urban, industrial, and agricultural sources, is enough to account for the retreat of Himalayan glaciers observed in the past half century. The glaciers supply water to major Asian rivers including the Yangtze, Ganges, and Indus. These rivers in turn comprise the chief water supply for billions of people in China, India, and other south Asian countries.
“The rapid melting of these glaciers, the third-largest ice mass on the planet, if it becomes widespread and continues for several more decades, will have unprecedented downstream effects on southern and eastern Asia,” the researchers concluded in the Nature article.
The scientists based their conclusions in large part on data gathered by a fleet of unmanned aircraft during a landmark field campaign conducted in March 2006 in the skies over the Maldives, an island nation in the Indian Ocean south of India. The Maldives Autonomous unmanned aerial vehicle Campaign (MAC) took place during the region’s dry season when polluted air masses travel south from the continent to the Indian Ocean. The air typically contains particles released from industrial and vehicle emissions as well as through biomass burning.
Such polluted air has been demonstrated to have a dual effect of warming the atmosphere as particles absorb sunlight and of cooling the earth’s surface as the particles curb the amount of sunlight that reaches the ground. The net effect of the two forces remains uncertain but other research by Ramanathan has suggested that the surface dimming might serve to mask global warming, leading scientists and the public to underappreciate the full magnitude of anthropogenic climate change.
The aircraft, flying in stacked formations, made nearly simultaneous measurements of the brown clouds from different altitudes, creating a profile of soot concentrations and light absorption that was unprecedented in its level of detail.
The researchers validated the data from the aircraft with ground-based measurements taken at a station at the Maldivian island Hanimadhoo.
When the researchers fed both greenhouse gas and brown cloud data into computer climate models, the simulations yielded an estimate that the region’s atmosphere has warmed 0.25 degrees C (0.5 degrees F) per decade since 1950 at altitudes ranging from 2 to 5 kilometers (6,500 to 16,500 feet) above sea level. At those heights are found many of the glaciers in the Himalayas. The amount of heating corresponds to observed levels of glacial retreat. The analysis revealed that the effect of the brown cloud was necessary to explain temperature changes that have been observed in the region over the last half-century. It also indicated that south Asia’s warming trend is more pronounced at higher altitudes than closer to sea level.
“Earth's climate profoundly affects virtually all sectors of human needs and activities. This study further demonstrates that our climate hangs in delicate balance between natural and man-made forces,” said Jay Fein, program director in the Division of Atmospheric Sciences at the National Science Foundation, which provided the main funding for the research.
— Robert Monroe