Satellites expose myth of marching Sahara - Sahara desert not expanding

Science News, July 20, 1991 by Richard Monastersky

The expansion of the Sahara desert appears a dramatic example of an environmental crisis: Picture in the media show towns burined under sand dunes that engulf more and more arable land eacy year. But satellite measurements over the last decade reveal the march of the Sahara--the largest desert on Earth -- is not as widespread or severe as the United Nations and other organizations had supposed.

Reports in the 1970s and 1980s portrayed the southern edge of the Sahara rolling ever southward like a wave, expanding into the sub-Saharan Sahel region at a rate of 5 kilometers per year. International organizations laid much of the blame of this "desertification" on overgrazing and other land use problems.

But vegetation measurements collected every day by U.S. meteorological satellites show that the southern edge of the Sahara ebbed and flowed more like a tide over an 11-year period starting in 1980. Compton J. Tucker and Wilbur W. Newcomb of NASA's Goddard Space Flight Center in Greenbelt, Md., and Harold E. Dregne of Texas Tech University in Lubbock report the observation in the July 19 SCIENCE.

"The message [of these results] is that a lot of what's been claimed about the so-called desertification of the Sahel is just incorrect," says Sharon E. Nicholson, a meteorologist at Florida State University in Tallahassee, who has studied rainfall patterns in Africa.

Previous reports of desert expansion have erred by assuming that trends observed in a few isolated locations were occurring across the entire continent, says Tucker. They also failed to take into account the effect of a drought that plagued the region during the 1970s and 1980s, causing vegetation patterns to shift, he adds.

Meteorological satellites, however, now allow broad regional tracking of the desert's boundary, which Tucker's group defines as the area that receives 200 millimeters of rainfall per year. The scientists gauged precipitation amounts indirectly from the amount of red light reflected off Earth's surface. Because chlorophyll in plants absorbs red light, desert areas reflect more red light than do vegetated regions. Areas of thicker plants correspond to areas that receive greater amounts of rainfall.

The boundary between the arid Sahara and the semi-arid Sahel region migrated appreciably each year during the study period. From 1980 to 1984, the desert expanded southward, with the boundary shifting 240 kilometers (km). But from 1984 to 1985, the trend reversed and the divider moved north by 110 km in a single year. It moved northward another 30 km the next year. In 1987, the boundary shifted back southward by 55 km, and northward 100 km in 1988. In 1989 and 1990, it shifted southward 77 km.

While the southern extent of the Sahara in 1990 reached 130 km further south than in 1980, that difference does not reflect a long term trend but rather a difference in the year-to-year rainfall, Tucker says. The results suggest researchers will have to measure over a number of decades in order to discern any long-term changes due to human activities, he contends.

During the last decade, reports of desert expansion in the Sahara prompted development programs to provide substantial funds for planting and irrigating rows of trees -- an expensive effort designed to stabilize sand dunes. But these agencies are now shifting their focus, in part because of the satellite measurements. "This is the first time that we have had a broad overview of the desert boundary issue," says Ridley Nelson, and agricultural economist with the World Bank in Washington, D.C. Development organizations see desert advancement as less of a problem and are instead addressing practices that lower the productivity of dry land areas, Nelson says.