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Huge dust plumes from
China cause changes in climate
By ROBERT LEE HOTZ
One tainted export from China can’t
be avoided in North America — air.
An outpouring of dust layered with man-made sulfates, smog,
industrial fumes, carbon grit and nitrates is crossing the Pacific
Ocean on prevailing winds from booming Asian economies in plumes
so vast they alter the climate. These rivers of polluted air can
be wider than the Amazon and deeper than the Grand Canyon.
“There are times when it covers the entire Pacific Ocean basin
like a ribbon bent back and forth,” said atmospheric physicist V.
Ramanathan at the Scripps Institution of Oceanography in La Jolla,
Calif.
On some days, almost a third of the air over Los Angeles and San
Francisco can be traced directly to Asia. With it comes up to
three-quarters of the black carbon particulate pollution that
reaches the West Coast, Dr. Ramanathan and his colleagues recently
reported in the Journal of Geophysical Research.
This transcontinental pollution is part of a growing global
traffic in dust and aerosol particles made worse by drought and
deforestation, said Steven Cliff, who studies the problem at the
University of California at Davis.
Aerosols — airborne microscopic particles — are produced naturally
every time a breeze catches sea salt from ocean spray, or a
volcano erupts, or a forest burns, or a windstorm kicks up dust,
for example. They also are released in exhaust fumes, factory
vapors and coal-fired power plant emissions.
Over the Pacific itself, the plumes are seeding ocean clouds and
spawning fiercer thunderstorms, researchers at Texas A&M
University reported in the Proceedings of the National Academy of
Sciences in March.
The influence of these plumes on climate is complex because they
can have both a cooling and a warming effect, the scientists said.
Scientists are convinced these plumes contain so many cooling
sulfate particles that they may be masking half of the effect of
global warming. The plumes may block more than 10% of the sunlight
over the Pacific.
But while the sulfates they carry lower temperatures by reflecting
sunlight, the soot they contain absorbs solar heat, thus warming
the planet.
Asia is the world’s largest source of aerosols, man-made and
natural. Every spring and summer, storms whip up silt from the
Gobi desert of Mongolia and the hardpan of the Taklamakan desert
of western China, where, for centuries, dust has shaped a way of
life. From the dunes of Dunhuang, where vendors hawk gauze face
masks alongside braided leather camel whips, to the oasis of
Kashgar at the feet of the Tian Shan Mountains 1,500 miles to the
west, there is no escaping it.
The Taklamakan is a natural engine of evaporation and erosion.
Rare among the world’s continental basins, no river that enters
the Taklamakan ever reaches the sea. Fed by melting highland
glaciers and gorged with silt, these freshwater torrents all
vanish in the arid desert heat, like so many Silk Road caravans.
Only the dust escapes.
In an instant, billows of grit can envelope the landscape in a
mist so fine that it never completely settles. Moving east, the
dust sweeps up pollutants from heavily industrialized regions that
turn the yellow plumes a bruised brown. In Beijing, where
authorities estimate a million tons of this dust settles every
year, the level of microscopic aerosols is seven times the
public-health standard set by the World Health Organization.
Once aloft, the plumes can circle the world in three weeks. “In a
very real and immediate sense, you can look at a dust event you
are breathing in China and look at this same dust as it tracks
across the Pacific and reaches the United States,” said climate
analyst Jeff Stith at the National Center for Atmospheric Research
in Colorado. “It is a remarkable mix of natural and man-made
particles.”
This spring, Dr. Ramanathan and Dr. Stith led an international
research team in a $1 million National Science Foundation project
to track systematically the plumes across the Pacific. NASA
satellites have monitored the clouds from orbit for several years,
but this was the first effort to analyze them in detail.
For six weeks, the researchers cruised the Pacific aboard a
specially instrumented Gulfstream V jet to sample these exotic
airstreams. Their findings, to be released this year, involved
NASA, the National Oceanic and Atmospheric Administration and nine
U.S. universities, as well as the National Institute for
Environmental Studies in Japan, Seoul National University in
Korea, and Lanzhou University and Peking University in China.
The team detected a new high-altitude plume every three or four
days. Each one was up to 300 miles wide and six miles deep, a
vaporous layer cake of pollutants. The higher the plumes, the
longer they lasted, the faster they traveled and the more
pronounced their effect, the researchers said.
Until now, the pollution choking so many communities in Asia may
have tempered the pace of global warming. As China and other
countries eliminate their sulfate emissions, however, world
temperatures may heat up even faster than predicted. |
