spread or encroachment of a desert environment into arid or semiarid regions, caused by climatic changes, human influence, or both. Climatic factors include periods of temporary but severe drought and long-term climatic changes toward aridity. Human factors include the artificial alteration of the climate, such as degradation of the biological environment in arid regions by removing vegetation (which can lead to unnaturally high erosion), excessive cultivation, and the exhaustion of surface-water or groundwater supplies for irrigation, industry, or domestic use.

Desertification drains an arid or semiarid land of its life-supporting capabilities. The process is characterized by a declining groundwater table (water table), salinization of topsoil and water, diminution of surface water, increasing erosion, and the disappearance of native vegetation. Areas undergoing desertification may show all of these symptoms, but the existence of only one usually provides sufficient evidence that the process is taking place. Desertification usually begins in areas made susceptible by drought or overuse by human populations and spreads into arid and semiarid regions.

Desertification is not limited to nondesert regions and can occur in areas within deserts where the delicate ecological balance is disturbed. The Sonoran (Sonoran Desert) and Chihuahuan deserts of the American Southwest, for example, have become observably more barren as the wildlife and plant populations have diminished.

Public awareness of desertification increased during the severe drought in the Sahel in Africa (1968–73), a drought that accelerated the southward movement of the Sahara (desert). In 1977 the worldwide consequences of desertification were the subject of a UN Conference on Desertification (UNCOD), held in Nairobi, Kenya.