Expanding populations are subjecting the land to increasing pressures to provide them with food and fuel. The semiarid lands bordering the deserts exist in a delicate ecological balance and are limited in their potential to adjust to increased environmental pressures. There is little doubt, however, that desertification in most areas results primarily from human activities rather than natural processes. The process may be accelerated in subsequent decades if global warming resulting from air pollution seriously increases. Continued gradual global warming has produced an increase in aridity for some areas over the past few thousand years. In some regions, the increase in desert areas is occurring largely as the result of a trend toward drier climatic conditions. The gradual drying of the soil caused by its diminished ability to absorb water results in the further loss of vegetation, so that a cycle of progressive surface deterioration is established. Water absorption is greatly reduced consequently runoff is increased, resulting in accelerated erosion rates. The impact of raindrops on the loose soil tends to transfer fine clay particles into the tiniest soil spaces, sealing them and producing a surface that allows very little water penetration. In other cases, the finer particles may be removed, while the sand-sized particles are accumulated to form mobile hills or ridges of sand.Д®ven in the areas that retain a soil cover, the reduction of vegetation typically results in the loss of the soil's ability to absorb substantial quantities of water. In some cases the loose soil is blown completely away, leaving a stony surface. It has been estimated that an additional one-fourth of the Earth's land surface is threatened by this process.Дesertification is accomplished primarily through the loss of stabilizing natural vegetation and the subsequent accelerated erosion of the soil by wind and water. The expansion of desert like conditions into areas where they did not previously exist is called desertification. The deserts, which already occupy approximately a fourth of the Earth's land surface, have in recent decades been increasing at an alarming pace.

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