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NEW
NASA SATELLITE SENSOR AND FIELD EXPERIMENT SHOWS AEROSOLS COOL THE SURFACE BUT
WARM THE ATMOSPHERE New
research based upon NASA satellite data and a multi-national field experiment
shows that black carbon aerosol pollution produced by humans can impact global
climate as well as seasonal cycles of rainfall. Because
aerosols that contain black carbon both absorb and reflect incoming sunlight,
these particles can exert a regional cooling influence on Earth's surface that
is about 3 times greater than the warming effect of greenhouse gases. But even
as these aerosols reduce by as much as 10 percent the amount of sunlight reaching
the surface, they increase the solar energy absorbed in the atmosphere by 50 percent,
thus making it possible to both cool the surface and warm the atmosphere. Scientists
are concerned that this heating may perturb atmospheric circulation and rainfall
patterns. "When
we combined the satellite measurements with surface measurements, we found that
the reduction of sunlight reaching the surface was three times larger than the
amount of sunlight reflected back to space," says V. Ramanathan, director
of the Center for Clouds, Chemistry, and Climate at Scripps Institution of Oceanography
at the University of California, San Diego. "Averaged over the entire northern
Indian Ocean, the man-made pollutants reflected more solar radiation back to space
(than pristine skies), but they absorbed up to twice as much radiation in the
atmosphere." Together
with K. Rajeev, of India's Vikram Sarabhai Space Centre, the authors report their
findings in the August 16 Journal of Geophysical Research. Data for their investigation
were collected during the Indian Ocean Experiment (INDOEX), an international,
multi-agency measurement campaign conducted from January through March in the
years 1997, 1998, and 1999. INDOEX
used instruments on land and on aircraft together with measurements made by NASA's
Clouds and Earth's Radiant Energy System (CERES) sensor as it flew overhead aboard
the Tropical Rainfall Measuring Mission (TRMM) satellite. The experiment's objective
was to help scientists understand to what extent human-produced aerosols may offset
global warming. Earlier global warming studies suggest aerosols make our world
"brighter" by reflecting more sunlight back to space, thereby helping
to counteract the greenhouse effect. The
Indian subcontinent offered the architects of the INDOEX campaign an ideal setting
for their field experiment. The region was chosen for its unique combination of
meteorology, landscape (relatively flat plains framed by the towering Himalayan
Mountains to the north and open ocean to the south), and the large Southern Asian
population (roughly 1.5 billion) with a growing economy. "Together,
these features maximize the effects of aerosol pollution," Ramanathan explains.
As a result of human industry, automobiles, factories, and burning vegetation,
particles build up in the atmosphere where they are blown southward over most
of the tropical Indian Ocean. The Indo-Asian haze covered an area larger than
that of the United States. Although the INDOEX team found atmospheric particles
of natural origin - such as trace amounts of sea salts and desert dust - they
also found that 75 percent of the aerosol over the region resulted from human
activities, including sulfates, nitrates, black carbon, and fly ash. Most natural
aerosols scatter and reflect sunlight back to space, thereby making our planet
brighter. However, human-produced black carbon aerosol absorbs more light than
it reflects, thereby making our planet darker. "Ultimately,
we want to determine if our planet as a whole is getting brighter or darker,"
Ramanathan states. "We could not answer that question until we could measure
the sunlight reflected at the top of the atmosphere with an absolute accuracy
of 1 percent. The CERES sensors provide that accuracy for the first time ever
from a space-based sensor." CERES
was first launched in 1997 aboard TRMM, which flies in a near-equatorial orbit.
Two more CERES sensors were launched in December 1999 aboard NASA's Terra satellite,
which flies in a near-polar orbit. Terra's polar orbit allows CERES to measure
the Earth's incoming and outgoing radiant energy on a global scale every day.
Moreover, the Moderate-resolution Imaging Spectroradiometer (MODIS) aboard Terra
makes precise global measurements of aerosols that greatly enhance scientists'
ability to study their effects. "A
large reduction of sunlight at the surface has implications for the hydrological
cycle because of the close tie between heat and evaporation," Ramanathan
says. "It could change the heating structure of the atmosphere and perturb
the climate system in ways we don't understand now. We don't know, for example,
how this might affect the monsoon season." While
Ramanathan admits that scientists don't know the net effect of bright and dark
aerosols on global climate, through INDOEX and CERES they have shown that aerosols
have a net cooling effect on the surface and they now know the magnitude of that
cooling over a large region. But, he says, the INDOEX campaign does not solve
the greater mystery. The next step is to use the CERES and MODIS sensors aboard
NASA's Terra satellite to extend this study to the global scale. The
study was funded by the National Science Foundation. Journalists
may request a copy of the paper from Harvey Leifert at hleifert@agu.org.
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