desertification

upright=1.4|thumb|Global distribution of Drylands|dryland areas based on the [[aridity index computed over a 30-year average during 1981 to 2010. Typical deserts are indicated by the hyper-arid category (light yellow).]] thumb|right|234px|Extreme desertification in Somalia Desertification is a type of gradual land degradation of fertile land into arid desert due to a combination of natural processes and human activities.

The immediate cause of desertification is the loss of most vegetation. This is driven by a number of factors, alone or in combination, such as drought, climatic shifts, tillage for agriculture, overgrazing and deforestation for fuel or construction materials. Though vegetation plays a major role in determining the biological composition of the soil, studies have shown that, in many environments, the rate of erosion and runoff decreases exponentially with increased vegetation cover. Unprotected, dry soil surfaces blow away with the wind or are washed away by flash floods, leaving infertile lower soil layers that bake in the sun and become an unproductive hardpan.