Desertification

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.

III.

CONSEQUENCES OF DESERTIFICATION

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