Cynthia M. O’Carroll, Goddard Space Flight Center
Greenbelt, MD (Phone: 301/614-5563)
Mario Aguilera or Cindy Clark, Scripps Institution of Oceanography
(Phone 858/534-3624)
RELEASE NO: 01-80
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 Veerabhadran 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.
###
For more information see:
http://www.gsfc.nasa.gov/topstory/20010816indoex.html
http://www.earthobservatory.nasa.gov/Library/Aerosols/
Journalists may request a copy of the paper from Harvey Leifert at
hleifert@agu.org. Please indicate whether you prefer PDF or fax and
provide your contact information.
Scripps Institution of Oceanography on the World Wide Web: scripps.ucsd.edu
Scripps News on the World Wide Web: scrippsnews.ucsd.edu |
|
High-Resolution Stills
72 dpi Comparison (72 KB JPEG)
300 dpi Comparison (2.0 MB TIFF)
Color Palettes (44 KB TIFF)
300 dpi Map (710 KB TIFF)
Animations:
Zoom and comparison (1.9 MB)
Sequence of 8-day images (3.3 MB)
Aerosols that contain black carbon both absorb and reflect incoming
sunlight. Even as these atmospheric particles reduce the amount of
sunlight reaching the surface, they increase the amount of solar energy
absorbed in the atmosphere, thus making it possible to both cool the
surface and warm the atmosphere. The animations above zoom into the
region of focus during the Indian Ocean Experiment (INDOEX)—a vast
region spanning the Arabian Sea and Bay of Bengal (west to east), and
from the foot of the Himalayan Mountains, across the Indian subcontinent
to the southern Indian Ocean (north to south).
The 8-day composite image above
shows aerosol pollution (brownish pixels) in the lower atmosphere over
the INDOEX study area, as measured by the Moderate-resolution Imaging
Spectroradiometer (MODIS) aboard Terra. These data were composited from
March 14-21, 2001.
The second image
shows the total solar energy reflected back to space, as measured
by the Clouds and Earth's Radiant Energy System (CERES) aboard Terra.
White pixels show high values, greens are intermediate values, and blues
are low. Note how the aerosols, particularly over the ocean, increase
the amount of energy reflected back to space.
The third image
shows the absorption of the black carbon aerosols in the
atmosphere. Where the aerosols are most dense, the absorption is
highest. Red pixels indicate the highest levels of absorption, blues are
low.
The
fourth image shows that the aerosol particles
reduce the amount of sunlight reaching the surface. Dark pixels show
where the aerosols exert their cooling influence on the surface (or a high magnitude of negative radiative forcing). The
bright pixels show where there is much less aerosol pollution and the
incoming sunlight is relatively unaffected.
Images and animations by Tom Bridgman, NASA GSFC Science Visualization Studio; Data courtesy Terra CERES and MODIS Science Teams, NASA LaRC and GSFC, and Scripps Institution of Oceanography |
