Desertification, decline in the biological
or economic productivity of the soil in arid and semiarid areas resulting from
various factors, including human activities and variations in climate. Desertification
refers to the formation and expansion of degraded soil, not to the
advancing movement of the current deserts. Desertification is found on every
continent except Antarctica, but international attention has focused mostly on
Africa, particularly the region known as the Sahel, the region of northern
Africa immediately to the south of the Sahara desert. Desertification has been
recognized as a problem since the Dust Bowl of the 1930s in the Midwestern
United States, but it only became an international issue during the Great
Drought in the Sahel between 1968 and 1973.
Desertification occurs in cropland (both irrigated and nonirrigated),
pasture, and woodlands. Loss of soil, deterioration of soil, and loss of
natural vegetation all lead to desertification. Drought, a period of unusually
dry weather, can cause loss of vegetation, which in turn leads to
desertification. Poor land management and increasing population are factors that
promote increased irrigation, improper cultivation or overcultivation, and
increased numbers of livestock. These events alter the land and the soil,
diminish the resources, and increase the chances of desertification.
Desertification has sometimes been mistakenly described as the
expansion of deserts into nondesert areas. Pictures of sand dunes engulfing
agricultural lands encourage this misconception, but this type of
desertification is rare. Arid and semiarid lands can be degraded even if there
is no adjacent desert. Drought has also mistakenly been called the primary
cause of desertification. Desertification can occur without drought, and
drought can occur without resulting in desertification. Droughts are short-term
and cyclical. By themselves, they do not degrade the land. However, they
intensify the pressures that lead to mismanagement of land, plant, and water
resources.
While dryland rainfall is low on average, it is extremely variable
from year to year and from place to place. Native plants and animals have
adapted to this variability. For example, the seeds of many desert plants can
remain dormant through several years of drought, waiting for a good rain to
sprout. In order to survive in this harsh environment, humans must adapt their
activities as well. However, many factors, including population growth,
poverty, politics, disrupted social institutions, and the pursuit of short-term
economic opportunity, may work together to promote unsustainable practices.
Ironically, the availability of water for irrigation can cause
desertification. Nearly all irrigation water contains some salt. If an
irrigation system lacks a good drainage system, then the salt accumulates in
the soil. Eventually, the salt reaches levels toxic to most plants. This
problem is now jeopardizing about one-third of the world’s irrigated land. For
example, in the Euphrates Valley of Syria, irrigation costs dropped with the
introduction of diesel pumps in the 1940s, encouraging development of new
croplands. By 1980 half the land area had extremely high salt concentrations
and a large part of the area had been abandoned. Similar events have occurred
in Queensland, Australia; in the Indus Valley of Pakistan; and in the Imperial
and Coachella valleys in California.
See Irrigation: Problems Arising
from Irrigation.
A political change in Mauritania was largely responsible for
the desertification that began there in the 1960s. Prior to Mauritania’s gain
of independence in 1960, Mauritanian pastoralists could freely move south into
Senegal during times of drought. The border established at the time that
independence was gained restricted these traditional movements. This blockage
increased competition for the limited forage available and resulted in
overgrazing during times of drought. The consequence was a general decline in
nomadic pastoralism, or livestock rearing, and an increase in Mauritania’s
urban population.
Cultivation of the land, especially overcultivation or the
introduction of nonnative plant species, can lead to the loss of topsoil and
degradation of the soil. Before the Dust Bowl in the 1930s, high wheat prices
and favorable climatic conditions in the Midwestern United States encouraged
early 20th-century settlers to convert native grasslands to cropland. The
native grasses had held the fine-grained soil in place in spite of recurrent
droughts and strong winds. The crops planted by the settlers, however, were
adapted to neither drought nor wind. When the drought came in the 1930s, the
crops failed. Without the cover of crops to protect the soil, the soil began to
blow away. In many places, 8 to 10 cm (3 to 4 in) of topsoil vanished in the
wind. When the rains returned, the farms were less productive due to the loss
of much of their nutrient-rich topsoil. The crop failures and subsequent
declines in production ruined many farms. As in other arid lands, many families
had few options other than migration. Those people who remained adopted new
soil conservation practices, and they were also protected by a new system of
government crop subsidies and insurance.
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CONSEQUENCES OF DESERTIFICATION
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In most cases of desertification,
there is a reduction in total species richness, an increase in the proportion
of exotic (nonnative) plants, and a decline in overall biodiversity—the variety
of life forms and the ecological roles they fill. Once desertification starts,
it often causes changes that accelerate the process. For example,
desertification often results in a decrease in the amount of vegetation
covering the land. With less vegetation providing shade, soil temperatures
rise, accelerating the breakdown of organic matter in the soil and the
evaporation of water. Some soils may become compacted or crusted, reducing
their ability to absorb the limited rainfall that occurs, which further reduces
the amount of water available for plants. The absence of vegetation also
enhances runoff and erosion by water and wind. Erosion may form deep gullies,
lowering the local water table (level of water within the ground) and making
less water available for plants. Wind erosion blows away nutrients in the soil
and may physically damage plants. Each of these effects makes plant growth more
difficult and may further reduce the amount of vegetation covering the land,
which in turn leads to more degradation.
For land managers, desertification
is a downward spiral. As it proceeds, the impact of any downturn, such as a
drought, may become catastrophic and result in loss of human lives due to lack
of necessary resources, such as water. With diminishing productivity and
profitability comes increased pressure to compensate for declines. Livestock
grazers increase herd sizes, and farmers plant all available plots of land and
continue to irrigate even though yields shrink. In the poorest regions where no
other employment is available, rural populations turn to woodcutting and
charcoal production, which lead to deforestation. This deforestation forces
families to spend more time seeking firewood for domestic use, leaving less
time for tending fields or animals.
Desertification has become
a large-scale problem. Arid and semiarid regions, known as drylands, account
for one-third of the world’s land area and support a combined population of
about 900 million people. Soil degradation reduces crop output and is a major
concern economically. About 70 percent of drylands are susceptible to
degradation, 50 percent have been degraded to some degree, and 15 percent show
extreme degradation where agricultural yields are less than half of their
former level. Almost all the areas of extreme degradation are in the African
Sahel from Senegal to Sudan, along the Mediterranean from Tunisia to Egypt, and
in central and southwestern Asia from China to Syria.
Desertification can also
have impacts that extend beyond the immediate degraded area. Wind-borne dust
from the Sahel creates havoc with air traffic across western Africa, and
sediment eroded from central China damages water control systems far downstream.
Many regions are affected indirectly by desertification as they absorb waves of
people uprooted by their inability to grow enough food or raise enough
livestock. Such people, called environmental refugees, swelled the urban areas
of the Sahel during the 1970s and 1980s. Moreover, migrations of environmental
refugees may cross national boundaries and contribute to political friction
within and between countries. This migration problem has occurred at various
times, such as during the drought years from 1968 to 1973 when Mauritanian
nomads fled into Senegal
