Parts of it look like the surface of Mars. Beginning in 1968, the Sahel—a vast, ancient savanna that borders the lush, tropical regions of West Africa—fell victim to a catastrophic five-year drought. Abnormally dry conditions sped up the invasion of a neighbor to the north: the Sahara, the world’s largest desert. When the grass- and shrubland of the Sahel lost its already limited ability to support crops and livestock, famine came to visit along with the sand.

Verdant land can become desert as a result of drought, increased erosion due to land-clearing, poor farming techniques, overgrazing of livestock, and drainage of surface and underground water for crop irrigation and household and industrial use.

Even an existing desert can become more of a wasteland when ecological balances change. The Sonoran and Chihuahuan Deserts of the U.S. Southwest and Mexico have become increasingly barren as native plants and wildlife have been diminished by several factors, including the depletion of groundwater by human activity.

Perhaps the most frightening aspect of desertification is that it tends to be what scientists call a runaway phenomenon. Once it begins in a particular area, it is almost impossible to stop, and it cannot be reversed within a human lifetime.