Saharan Dust "Cools" Climate Warming Estimates

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.