Asian Forecast: Hazy, Warmer—Clouds of pollution heat lower atmosphere

The murky clouds of smoke and soot that blanket many regions of Asia have heated the lower atmosphere there in recent decades as much as increases in carbon dioxide and other greenhouse gases have, a new field study suggests.

RIVER OF HAZE. Dense “brown clouds” of pollution that smother much of Asia, such as the gray haze wafting over northern India’s Ganges River in this Dec. 17, 2004, image, boost the lower atmosphere’s temperature, a new study suggests. J. Allen/NASA

Scientists have long argued about the net climatic effect of aerosols such as dust, smoke, and soot. Light-colored aerosols scatter much of the light that strikes them, some of it back to space, says V. Ram Ramanathan, a climate scientist at the Scripps Institution of Oceanography in La Jolla, Calif. However, dark aerosols such as soot can absorb much of the incoming radiation, warming themselves and the air around them. Current estimates of the overall effect of light-dark mixtures—including the so-called atmospheric brown clouds of pollution found in parts of Asia—are based largely on computer simulations, says Ramanathan.

Now, he and his colleagues weigh in on the debate. In March 2006, they repeatedly steered a squadron of instrumented drones through clouds of pollution wafting over Hanimadhoo, a remote island in the Maldives archipelago southwest of India’s southern tip. During the flights, the scientists measured temperature, humidity, and intensity of sunlight at various wavelengths nearly simultaneously at several altitudes over the island. They also gathered data at a land-based weather station.

During the last 2 weeks of the field test, winds were bringing air masses from India to the island, says Ramanathan. On those days, the array of data suggests that each cubic centimeter of air between the altitudes of 1 kilometer and 3 km contained about 2,500 particles of smoke and soot. Overall, the temperature of the air between altitudes of 500 meters and 3 km was about 0.5°C warmer than it would have been without the pollution, the researchers estimate. About 90 percent of that heating can be attributed to soot, they report in the Aug. 2 Nature.

Between 1950 and 2000, brown clouds warmed the lower atmosphere’s yearly average temperature as much as 0.8°C in the region, the team estimates. During the same period, increased atmospheric concentrations of greenhouse gases such as carbon dioxide had a comparable effect, says Ramanathan. Overall, the lower atmosphere in the region has warmed about 0.25°C each decade since 1950, causing major melting of many Himalayan glaciers.

“Scientists used to think of atmospheric brown clouds as masking global warming” by cooling the air at ground level, Ramanathan notes. “Our new findings show that [brown clouds and greenhouse gases] actually are working together” to heat the atmosphere.

Results of this field study demonstrate that similar initiatives using airborne drones are essential for advancing climate research, says Peter Pilewskie, an atmospheric scientist at the University of Colorado at Boulder. The data from such research, as well as those gathered by aerosol-detecting satellites, will enable scientists to better assess the effect of airborne particles on global climate, he notes in a comment appearing with the new study.

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