Goddard Space Flight Center
 Desert
dust may slightly diminish estimates on how warm the world will become, based
on findings of how much sunlight is absorbed by dust.
Scientists studying
dust blowing off the Sahara Desert have found that dust particles absorb much
less solar radiation than previously thought, reducing the amount of solar warming
of the Earth's surface. These results appear in the April 15 issue of the American
Geophysical Union's Journal of Geophysical Research Letters.
"Recent studies
have suggested that desert dust absorbs 10-15 percent of the sunlight that hits
the dust particles, in the visible to near infrared (IR) part of the solar spectrum,"
says lead author Yoram J. Kaufman of NASA's Goddard Space Flight Center, Greenbelt,
Md. "These studies have large uncertainties, and are based on old laboratory measurements
of dust that were shown to be inaccurate. Our new results, produced with two independent
sets of remote observations, found dust absorption to be 1-5 percent." This means
that the Earth's surface receives less warmth in areas where dust lingers in the
atmosphere, because that radiation is reflected back into space.
The Saharan
dust absorption results are likely to be representative of desert dust properties
around the world, according to the researchers. "The new results strongly suggest
that mineral dust from other regions of the world will also be less absorbing
than previously thought," says co-author Lorraine Remer of Goddard. "So, more
dust in the atmosphere will lower current estimates of warming temperatures in
those areas." Remer has analyzed Asian dust that reached the Channel Islands off
the southern California coast and found similar low levels of absorption.
Previous
estimates of how much sunlight is absorbed and reflected by desert dust have varied
so widely - some show a net warming effect on the atmosphere while others show
a net cooling - that they resulted in both possibilities for warming and cooling
in the climate projections issued earlier this year by the Intergovernmental Panel
on Climate Change.
The researchers are confident that desert dust absorbs
far less radiation than previously thought, Kaufman says, despite the difficulty
of measuring absorption in dust. Kaufman said two independent methods and observations
were used, and both measure the properties of dust as it is in the free atmosphere,
not after collecting it into a measuring device.
One approach used satellite
observations from 1987 of a dust storm over Senegal along the coast of western
Africa, as measured by a French science team. The scientists compared two images
from NASA's Landsat 5 spacecraft taken two weeks apart, one during an intense
dust storm and another when dust levels were much lower. The difference in the
brightness of solar radiation reflected by the land surface and the heavy dust
cloud indicated that nearly all the sunlight in the visible and near IR part of
solar spectrum hitting the dust cloud was reflected back into space. Very little
was absorbed by the iron-rich dust particles, since the absorption takes place
only in the blue and ultraviolet wavelengths.
A very similar ratio of reflected
sunlight to absorbed sunlight was seen in the same area 12 years later by looking
up from the ground through the dust-filled sky. Instruments on Cape Verde Islands,
off the Senegalese coast, recorded sunlight reaching the surface over the summer
of 1999. From the unique patterns of light observed, the scientists could infer
the size of the dust particles and their absorbing properties.
The sun
photometers used on the Cape Verde Islands are part of the worldwide Aerosol Robotic
Network (AERONET), a ground-based monitoring network and data archive supported
by the French space and research organizations (CNES & CNRS) and NASA's Earth
Observing System.
These findings are the result of a collaboration of scientists
at GSFC, CNRS/University of Lille and the Israeli Institute for Desert Research/
Ben-Gurion University.
"Absorption of Sunlight by Dust as Inferred from
Satellite and Ground-based Remote Sensing," by Y. J. Kaufman, Didier Tanre, O.
Dubovik, A. Karnieli, L.A. Remer, appears in Geophysical Research Letters, April
15, 2001.
The Landsat program is part of NASA's Earth Science Enterprise,
a long-term research program designed to study the Earth's land, oceans, air,
ice and life as a total system. Landsat 5 was launched by NASA in 1984 and is
still collecting images. The Space Imaging Corporation of Denver, Colorado, maintains
Landsat 5 operations. |
