The deserts, which already occupy approximately a fourth of the Earth's land surface,
have in recent decades been increasing at an alarming pace. The expansion of desert-like
conditions into areas where they did not previously exist is called desertification. It
has been estimated that an additional one-fourth of the Earth's land surface is threatened
by this process.
Desertification is accomplished primarily through the loss of stabilizing natural vegetation
and the subsequent accelerated erosion of the soil by wind and water. In some cases the
loose soil is blown completely away, leaving a stony surface. In other cases, the finer
particles may be removed, while the sand-sized particles are accumulated to form mobile
hills or ridges of sand.
Even 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. 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.
Water absorption is greatly reduced; consequently runoff is increased, resulting in
accelerated erosion rates. 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.
In some regions, the increase in desert areas is occurring largely as the result of
a trend toward drier climatic conditions. Continued gradual global warming has produced
an increase in aridity for some areas over the past few thousand years. The process may
be accelerated in subsequent decades if global warming resulting from air pollution
seriously increases.
There is little doubt, however, that desertification in most areas results primarily
from human activities rather than natural processes. The semiarid lands bordering the
deserts exist in a delicate ecological balance and are limited in their potential to
adjust to increased environmental pressures. Expanding populations are subjecting the
land to increasing pressures to provide them with food and fuel. In wet periods, the
land may be able to respond to these stresses. During the dry periods that are common
phenomena along the desert margins, though, the pressure on the land is often far in
excess of its diminished capacity, and desertification results.
Four specific activities have been identified as major contributors to the desertification
processes: overcultivation, overgrazing, firewood gathering, and overirrigation. The
cultivation of crops has expanded into progressively drier regions as population
densities have grown. These regions are especially likely to have periods of severe
dryness, so that crop failures are common. Since the raising of most crops necessitates
the prior removal of the natural vegetation, crop failures leave extensive tracts of
land devoid of a plant cover and susceptible to wind and water erosion.
The raising of livestock is a major economic activity in semiarid lands, where grasses
are generally the dominant type of natural vegetation. The consequences of an excessive
number of livestock grazing in an area are the reduction of the vegetation cover and the
trampling and pulverization of the soil. This is usually followed by the drying of the
soil and accelerated erosion.
Firewood is the chief fuel used for cooking and heating in many countries. The increased
pressures of expanding populations have led to the removal of woody plants so that many
cities and towns are surrounded by large areas completely lacking in trees and shrubs.
The increasing use of dried animal waste as a substitute fuel has also hurt the soil
because this valuable soil conditioner and source of plant nutrients is no longer
being returned to the land.
The final major human cause of desertification is soil salinization resulting from
overirrigation. Excess water from irrigation sinks down into the water table. If no
drainage system exists, the water table rises, bringing dissolved salts to the surface.
The water evaporates and the salts are left behind, creating a white crustal layer that
prevents air and water from reaching the underlying soil.
The extreme seriousness of desertification results from the vast areas of land and the
tremendous numbers of people affected, as well as from the great difficulty of reversing
or even slowing the process. Once the soil has been removed by erosion, only the passage
of centuries or millennia will enable new soil to form. In areas where considerable soil
still remains, though, a rigorously enforced program of land protection and cover-crop
planting may make it possible to reverse the present deterioration of the surface.