Moving Deserts Can Deceive

Article excerpt

DESERTS on the march. Sahara advances. It's a standard environmental alarm.

But is it true?

The Sahara has been caught creeping southward on occasions. And there are instances around the world where overgrazing and other human impacts have created wastelands in semiarid areas where plants once grew.

As so often happens in earth science, however, truth is more likely to be found in observing research programs covering large areas over long periods of time than it is in dramatic localized cases. Deserts have yet to receive such definitive study. Meanwhile, appearances can deceive.

For example, the southern edge of desert vegetation in western Sudan in 1975 apparently lay 90 to 100 kilometers (56 to 62 1/2 miles) south of its 1958 perimeter. Naive averaging suggested a southward creep of around 5.5 km (3.5 miles) a year. It was a dramatic statistic that found its way into news reports. Yet the same area showed no sign of such desert expansion in 1984.

It turns out that the Sahara's southern boundary migrates both north and south. Comparing its positions for two widely spaced years in one region doesn't tell you much about desertification.

Such spot studies don't detect the underlying year-to-year pattern. That pattern shows up clearly in a new study, which covers the entire southern Sahara boundary from 1980 to 1990.

Compton J. Tucker and Wilbur W. Newcomb at the National Aeronautics and Space Administration's Goddard Space Flight Center in Greenbelt, Md., and Harold E. Dregne of Texas Tech University in Lubbock, Texas, took advantage of the overview provided by American polar-orbiting weather satellites.

Those satellites record red and infrared light reflected from the ground. In their research paper, recently published in Science, the three analysts explain how they used these data to locate the transition zone between desert growth and more rain-dependent vegetation. …

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