Soot is Key Player in Himalayan Warming; Looming Water Woes in Asia
A NASA-led study has found new evidence that a "heat pump" effect, driven by emissions of soot, or black carbon, contributes as much (or more) to atmospheric warming in the Himalayas as greenhouse gases. This warming fuels the melting of glaciers and could threaten fresh water resources in a region that is home to more than a billion people.
Tiny, dark-colored aerosols — specifically black carbon — travel along wind currents from Asian cities and accumulate over the Tibetan Plateau and Himalayan foothills. These brown clouds of soot absorb sunlight, creating a layer of warm air that rises to higher altitudes. The air masses amplify the melting of glaciers and snow, and set in motion a cycle that draws yet more warm air to the region from the Indian Ocean.
"Over areas of the Himalayas, the rate of warming is more than five times faster than warming globally," said William Lau, head of atmospheric sciences at NASA’s Goddard Space Flight Center in Greenbelt, Md. "Based on the differences it’s not difficult to conclude that greenhouse gases are not the sole agents of change in this region. There’s a localized phenomenon at play."
Lau and colleagues discussed the results today at a press briefing in San Francisco at the Fall Meeting of the American Geophysical Union. A related modeling study by Lau and colleagues has been submitted to Environmental Research Letters for publication.
Nicknamed the "Third Pole" of Earth, the Tibetan Plateau holds the third largest amount of stored fresh water on the planet after the North and South Poles. The Himalayas, which dominate the Plateau, are the source of melt water for many of Asia’s most important rivers.
But since the 1960s, the acreage covered by Himalayan glaciers has declined by more than 20 percent. Some Himalayan glaciers are melting so rapidly, some scientists postulate, that they may vanish by mid-century if trends persist. Climatologists have generally blamed greenhouse gases for the retreat, but Lau's work suggest that may not be the complete story.
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 exhaust have thickened concentrations of black carbon in the air.
Rapid melting from this black-carbon fueled "heat pump" is occurring primarily in the western Tibetan Plateau, beginning each year in April and extending through early fall. The brisk melting coincides with the time when concentrations of aerosols like soot and dust are most dense in the atmosphere.
Building on work by Veerabhardran Ramanathan of the Scripps Institution of Oceanography, San Diego, Calif., Lau and colleagues conducted modeling experiments that simulated the movement of air masses in the region from 2000 to 2007. They also made detailed numerical analyses of how soot particles and other aerosols absorb heat from the sun.
"Field campaigns with ground observations are already underway with more planned to test Lau’s modeling results," said Hal Maring who manages the Radiation Sciences program at NASA Headquarters in Washington. "But even at this stage we should be compelled to take notice.”
"The science suggests that we’ve got to better monitor the flue on our 'rooftop to the world," said Lau. "For Asia's sake, we need to add another topic to the climate dialogue."
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Goddard Space Flight Center, Greenbelt, Md.