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Press Release
07-087
"Brown Cloud"
Particulate Pollution Amplifies Global Warming
Jeopardizes Asian water supplies, contributes to
Himalayan glacier melt
August 1, 2007
Scientists have concluded that the global warming trend
caused by the buildup of greenhouse gases is a major
contributor to the melting of Himalayan and other tropical
glaciers. Now, a new analysis of pollution-filled "brown
clouds" over south Asia by researchers at the Scripps
Institution of Oceanography in La Jolla, Calif., offers hope
that the region may be able to arrest some of the alarming
retreat of such glaciers by reducing its air pollution.
The team, led by atmospheric chemist V. Ramanathan of
Scripps, found that atmospheric brown clouds enhanced solar
heating of the lower atmosphere by about 50 percent. The
results are in a paper in this week's issue of the journal
Nature.
The combined heating effect of greenhouse gases and brown
clouds, which contain soot, trace metals and other particles
from urban, industrial and agricultural sources, is enough to
account for the retreat of Himalayan glaciers in the past half
century, the researchers concluded.
The glaciers supply water to major Asian rivers, including
the Yangtze, Ganges and Indus. These rivers are the chief
water supply for billions of people in China, India and other
south Asian countries.
"If it becomes widespread and continues for several more
decades, the rapid melting of these glaciers, the
third-largest ice mass on the planet, will have unprecedented
effects on southern and eastern Asia," said Ramanathan.
"In order to understand the processes that can throw
climate out of balance, Ramanathan and his colleagues, for the
first time ever, used small and inexpensive unmanned aircraft
as a creative means of sampling within and from all sides of
clouds," said Jay Fein, program director in the National
Science Foundation (NSF)'s Division of Atmospheric Sciences,
which funded the research. "These measurements led to these
remarkable results."
The scientists based their conclusions on data gathered by
a fleet of unmanned aircraft during a landmark field campaign
conducted in March 2006, in the skies over the Maldives, an
island nation in the Indian Ocean south of India.
The Maldives Autonomous unmanned aerial vehicle Campaign
(MAC) took place during the region's dry season when polluted
air masses travel south from the continent to the Indian
Ocean. The air typically contains particles released from
industrial and vehicle emissions, and through biomass
burning.
Such polluted air has a dual effect of warming the
atmosphere as particles absorb sunlight, and of cooling
Earth's surface as the particles reduce the amount of sunlight
that reaches the ground.
The aircraft, flying in stacked formations, made nearly
simultaneous measurements of brown clouds from different
altitudes, creating a profile of soot concentrations and light
absorption that was unprecedented in its level of vertical
detail.
The researchers validated the data from the aircraft with
ground-based measurements taken at a station at the Maldivian
island Hanimadhoo.
"The main cause of climate change is the buildup of
greenhouse gases from the burning of fossil fuels," said Achim
Steiner, United Nations under-secretary general and executive
director of the UN Environment Programme (UNEP), which helped
support the research. "But brown clouds, whose environmental
and economic impacts are beginning to be unraveled by
scientists, are complicating and in some cases aggravating
their effects.
"It is likely that in curbing greenhouse gases we can
tackle the twin challenges of climate change and brown clouds,
and in doing so, reap wider benefits--from reduced air
pollution to improved agricultural yields."
The analysis reveals that the effect of the brown clouds
explains temperature changes observed in the region over the
last half-century. It also indicates that south Asia's warming
trend is more pronounced at higher altitudes.
"The conventional thinking is that brown clouds have masked
as much as 50 percent of global warming by greenhouse gases
through so-called global dimming," said Ramanathan. "While
this is true globally, this study reveals that over southern
and eastern Asia, the soot particles in the brown clouds are
in fact amplifying the atmospheric warming trend caused by
greenhouse gases by as much as 50 percent."
In addition to Ramanathan, the report's authors include
Muvva Ramana, Gregory Roberts, Dohyeong Kim, Craig Corrigan,
Chul Chung from Scripps Oceanography and David Winkler from
NASA's Langley Research Center.
The National Ocean and Atmospheric Administration (NOAA)
and NASA also provided funding, as did UNEP, which sponsors
the Atmospheric Brown Clouds (ABC) project and operates the
Maldives ABC observatory in collaboration with Scripps.
-NSF-
Media Contacts
Cheryl Dybas, NSF (703)
292-7734 cdybas@nsf.gov
Rob Monroe,
SIO/UCSD (858) 534-3624 rmonroe@ucsd.edu
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