Their combined effect could be warming the lower atmosphere in the region by as much as 0.25 degrees Celsius per decade, which “may be sufficient to account for the observed retreat of the Himalayan glaciers,” noted a team of scientists led by V. Ramanathan of the Scripps Institution of Oceanography, California.

Such warming held “substantial implications for the elevated region of the Himalaya, where observed warming of 0.15-0.3 degrees Celsius during the past several decades has led to the rapid reduction of glacier mass,” said Peter Pilewskie of the University of Colorado at Boulder, U.S., in a commentary published in the same issue of the journal.

Fine particles in the atmosphere, known as aerosols, block some of the sunlight from reaching the ground and thereby cause a cooling of the earth’s surface. Consequently, the fourth assessment report of the Intergovernmental Panel on Climate Change released earlier this year found that the presence of aerosols could offset some of the global warming.

But particles like soot also absorb sunlight and consequently warm the surrounding atmosphere. Prof. Ramanathan and his team used three small unmanned aerial vehicles equipped with miniaturised instruments, which were flown together from an island in the Maldives, to make measurements above, inside and below a cloud containing aerosols during March last year. From these measurements, the amount of sunlight being absorbed by the ‘atmospheric brown cloud’ could be worked out.

“We found that the brown cloud was enhancing [the light absorbed in the atmosphere] by quite a bit,” said Prof. Ramanathan in a telephone interview.

Data from NASA’s Cloud Aerosol Lidar and Infrared Pathfinder Satellite (CALIPSO) showed that the cloud seen over the Arabian Sea was about 3 km thick and it stretched all the way across South Asia, including the Himalayan region. The East Asian brown cloud was even thicker, he said.

Brown clouds contained a variety of aerosols, including sulphates from coal combustion, nitrates produced by vehicular emissions as well as soot resulting from fossil fuel combustion and the burning of biomass, he remarked. The absorbing aerosol was mainly soot.

Using a climate model, it was found that the brown cloud “nearly doubled the warming trend” caused by the greenhouse gas increase, said Prof. Ramanathan.

It took nearly four years of effort to miniaturise the instruments that the unmanned aerial vehicles carried aloft, said Muvva V. Ramana, second author of the paper, who took his doctorate while with the Indian Space Research Organisation’s Space Physics Laboratory in Thiruvananthapuram.

Aerosols deposited on mountainsides around the Tibetan plateau might affect monsoon rainfall patterns over India, according to simulations carried out by William Lau of the NASA’s Goddard Space Flight Center and others.

In a talk at a recent conference on monsoon, organised by the Indian Institute of Science in Bangalore, Dr. Lau pointed out that dust particles transported by winds from Saudi Arabia, Pakistan and the Thar desert as well as soot could pile up on the foothills of the Himalayas. Soot was very absorbing of sunlight and dust only moderately so, he told this correspondent. But if the tiny soot particles got deposited on the larger dust particles, the light absorption by dust would increase dramatically.

Simulations using a general circulation model suggested that the absorbing aerosols deposited along the Himalayan mountains could produce considerable atmospheric heating over the Tibetan plateau and set off a process that increased rainfall over northern India and reduced rains over the southern part.

But this result needed to be validated with many more model studies as well as with observations, he cautioned.

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