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Soot’s Dominant Role in the Himalayas

Dr David Whitehouse

Tiny soot particles, also known as black carbon, are released into the air from the large South Asian cities. When fossil fuels are burned without enough oxygen to complete combustion, one of the byproducts is black carbon, an aerosol that absorbs solar radiation (most classes of aerosols typically reflect incoming sunlight, causing a cooling effect.) Rising populations in Asia, industrial and agricultural burning, and vehicle exhausts have increased concentrations of black carbon in the air.

New modelling studies indicate that they accumulate over a climate hotspot called the Tibetan Plateau and contribute as much (or more) to atmospheric warming in the Himalayas as greenhouse gases. “Over areas of the Himalayas, the rate of warming is more than five times faster than warming globally,” says William Lau, head of atmospheric sciences at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

Rapid melting in the region occurs primarily in the western Tibetan Plateau, beginning each year in April and extending through to Autumn. NASA scientists noticed that the melting occurs when concentrations of aerosols like soot and dust transported from places such as India and Nepal are at their densest in the atmosphere.

It seems that black carbon travels east along wind currents attached to dust and becomes trapped in the air against Himalayan foothills. The particles’ dark colour absorbs solar radiation, creating a layer of warm air from the surface that rises to higher altitudes above the mountain ranges causing glacier and snow melt.

The Himalayas hold the third largest amount of stored water on Earth after the polar regions. According to some studies, since the early 1960s, the area covered by Himalayan glaciers has declined by over 20 percent. Climatologists and campaigners have generally blamed greenhouse gases but this new work suggests that may not be the complete story.

Aerosols stay in the atmosphere for a far shorter period than greenhouse gasses so reductions in black carbon emission, if it could be achieved, would have a swifter regional impact than greenhouse gas reductions. This is an important factor because environmental campaigners often highlight future water resources problems in the area. The Himalayas are the source of meltwater for many of Asia’s most important rivers—the Ganges and Indus in India, the Brahmaputra in Bangladesh, the Salween through China, Thailand and Burma, the Mekong across Laos, Cambodia and Vietnam, and the Yellow and Yangtze rivers in China. Reducing pollution might be more cost effective than reducing greenhouse gasses in the short term.