"So many disparate elements, both natural and man-made, converge
in the Himalayas," said William Lau, a climatologist from NASA's
Goddard Space Flight Center in Greenbelt, Md. "There's no other
place in the world that could produce such a powerful atmospheric
heat pump," referring to a new hypothesis he's put forward to
explain the rapid retreat of Himalayan glaciers in recent
decades.
The Himalayas, home to the tallest mountains on Earth, include
more than 110 peaks and stretch 2,500 kilometers (1,550 miles).
Bounded to the north by the Tibetan Plateau, to the west by
deserts, and to the south by a bowl-like basin teeming with people,
the mountains hold 10,000 glaciers.
These massive rivers of ice spill off mountain sides and grind
down through creviced valleys. In the spring, when the monsoon
carries moist air from the Indian Ocean, the glaciers begin to
thaw, replenishing lakes, streams, and some of Asia's mightiest
rivers, on which more than a billion people depend.
South of the Himalayas -- which forms the east-west edge of the
table-like Tibetan Plateau -- the mountains give way to the
Indo-Gangetic plain, one of the most fertile and densely populated
areas on Earth. The plain has become a megalopolis of cities
including Delhi, Dhaka, Kanpur, and Karachi, as well as a hotspot
for air pollution, with a steady supply of industrial soot mixing
with ash and other particles in the air.
To the west, in the northwestern part of the Indian
subcontinent, the Thar Desert stretches across 200,000 square
kilometers (77,000 square miles) of arid, dusty land. During the
spring, westerly winds pluck dust and sand from the Thar and blow
it toward the Indo-Gangetic plain.
The dust joins a mash of industrial pollutants to create a
massive brown cloud visible from space. Underneath the brown cloud,
some solar radiation is blocked from reaching the surface, causing
the under-lying land surface to cool.
"Surprisingly, these brown aerosol clouds seem to have potent
climate consequences that affect the entire region," Lau said.
The thick soot and dust layer absorbs solar radiation, and heats
up the air around the Himalayan foothills. The warm, rising air
enhances the seasonal northward flow of humid monsoon winds,
forcing moisture and hot air up the slopes of the Himalayas.
As the aerosol particles rise on the warm, convecting air, they
produce more rain over northern India and the Himalayan foothill,
which further warms the atmosphere and fuels a "heat pump" that
draws yet more warm air to the region.
"The phenomenon changes the timing and intensity of the monsoon,
effectively transferring heat from the low-lying lands over the
subcontinent to the atmosphere over the Tibetan Plateau, which in
turn warms the high-altitude land surface and hastens glacial
retreat," Lau said. His modeling shows that aerosols --
particularly black carbon and dust -- likely cause as much of the
glacial retreat in the region as greenhouse gases via this "heat
pump" effect.
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