RELEASE NO: 98-189

RAINFALL, NOT HUMANS, CONTROLS THE SIZE OF THE SAHARA

The decades-old concern that land misuse by humans is causing the African Sahara desert to extend south into fertile land is not occurring, according to Earth scientists from the NASA Goddard Space Flight Center in Greenbelt, Md. and Florida State University.

Looking at nearly two decades of satellite observations of vegetation growing near the Sahara, Dr. Compton "Jim" Tucker, a Goddard biologist, and Dr. Sharon Nicholson, a Florida State meteorologist, found that although the southern border of the Sahara ebbs and flows from year to year, there has been no overall growth in the desert’s size.

In the 1970s and early 1980s, scientists had the impression that the desert was steadily moving south into the temperate zones. But according to Tucker, satellite data show that Saharan/Sahelian boundary rebounds to the north with rainfall in wet years such as 1994.

"We’re not saying that there is no land degradation," said Nicholson, who has studied the problem for over 20 years. "But this paradigm about the marching sands and the vegetation going away is not happening."

An area of semi-arid grassland called the "Sahel," lines the Sahara along its southern border. The Sahelian/Sahara border has received a lot of attention from scientists and policy makers because in very dry years, the border moves south.

"The interesting question is how the desert changes through time, and is it changing through time," he said. Tucker studied data collected from 1980 to 1997 by satellite sensors aboard National Oceanic and Atmospheric Administration (NOAA) Advanced Very High-Resolution Radiometer (AVHRR) meteorological satellites.

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Tucker and Nicholson found that there was significant year-to-year variation in rainfall, with an especially dry year in 1984, making it seem that during drought years the desert was expanding southward. After 1984, the Sahara border rebounded and was especially far north in 1994.

The AVHRR sensor aboard the NOAA satellites measures the infrared and near-infrared radiation reflected by plants. From the two wavelengths of radiation, a "greenness index" can be determined. The "greenness index" shows scientists what areas of ground are covered by vegetation, and Tucker has shown that vegetation cover corresponds to the amount of rainfall in a particular region.

Over the last 20 years, the Saharan boundary has followed rainfall trends, and Nicholson’s analysis of historical data, recently published in the American Meteorological Society’s Bulletin, confirms this research. "It’s virtually impossible to separate desertification from drought," she said.

Desertification is defined as the man-made or natural formation of desert from what once was usable land.

This study on desertification is sponsored by NASA and the U.S. Agency for International Development. NASA funded this research as part of its Earth Science Enterprise, a long-term research program designed to study the Earth’s land, oceans, air, ice and life as a total system.

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