desertification

desertification, the process by which natural or human causes reduce the biological productivity of drylands (arid and semiarid lands). Declines in productivity may be the result of climate change, deforestation, overgrazing, poverty, political instability, unsustainable irrigation practices, or combinations of these factors. The concept does not refer to the physical expansion of existing deserts but rather to the various processes that threaten all dryland ecosystems, including deserts as well as grasslands and scrublands.

The global reach of desertification

Slightly less than half of Earth’s ice-free land surface—approximately 52 million square km (about 20 million square miles)—is drylands, and these drylands cover some of the world’s poorest countries. The United Nations Environment Programme (UNEP) notes that desertification has affected 36 million square km (14 million square miles) of land and is a major international concern. According to the United Nations Convention to Combat Desertification, the lives of 250 million people are affected by desertification, and as many as 135 million people may be displaced by desertification by 2045, making it one of the most severe environmental challenges facing humanity.

Africa is the continent most affected by desertification, and one of the most obvious natural borders on the landmass is the southern edge of the Sahara desert. The countries that lie on the edge of the Sahara are among the poorest in the world, and they are subject to periodic droughts that devastate their peoples. African drylands (which include the Sahara, the Kalahari, and the grasslands of East Africa) span 20 million square km (about 7.7 million square miles), some 65 percent of the continent. One-third of Africa’s drylands are largely uninhabited arid deserts, while the remaining two-thirds support two-thirds of the continent’s burgeoning human population. As Africa’s population increases, the productivity of the land supporting this population declines. Some one-fifth of the irrigated cropland, three-fifths of the rain-fed cropland, and three-fourths of the rangeland have been at least moderately harmed by desertification.

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Causes and consequences of desertification

In general, desertification is caused by variations in climate and by unsustainable land-management practices in dryland environments. By their very nature, arid and semiarid ecosystems are characterized by sparse or variable rainfall. Thus, climatic changes such as those that result in extended droughts can rapidly reduce the biological productivity of those ecosystems. Such changes may be temporary, lasting only a season, or they may persist over many years and decades. On the other hand, plants and animals are quick to take advantage of wetter periods, and productivity can rapidly increase during these times.

Since dryland environments are used for a variety of human purposes (such as agriculture, animal grazing, and fuelwood collection), the various activities undertaken in them can exacerbate the problem of desertification and bring about lasting changes to dryland ecosystems. In 1977, at the United Nations Conference on Desertification (UNCOD) in Nairobi, Kenya, representatives and delegates first contemplated the worldwide effects of desertification. The conference explored the causes and contributing factors and also possible local and regional solutions to the phenomenon. In addition, the delegates considered the varied consequences of desertification, such as crop failures or decreased yields in rain-fed farmland, the loss of perennial plant cover and thus loss of forage for livestock, reduced woody biomass and thus scarcity of fuelwood and building materials, a decrease in potable water stocks from reductions in surface water and groundwater flow, increased sand dune intrusion onto croplands and settlements, increased flooding due to rising sedimentation in rivers and lakes, and amplified air and water pollution from dust and sedimentation.

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Ecosystems

Four areas affected by desertification

To better understand how climatic changes and human activities contribute to the process of desertification, the consequences listed above can be grouped into four broad areas:

1. Irrigated croplands, whose soils are often degraded by the accumulation of salts.
2. Rain-fed croplands, which experience unreliable rainfall and wind-driven soil erosion.
3. Grazing lands, which are harmed by overgrazing, soil compaction, and erosion.
4. Dry woodlands, which are plagued by the overconsumption of fuelwood.

Irrigated croplands

Nearly 2,750,000 square km (about 1,062,000 square miles) of croplands are irrigated. Over 60 percent of these irrigated areas occur in drylands. Certainly, some dryland areas have been irrigated for millennia, but other areas are more fragile. Of the irrigated dryland, 30 percent (an area roughly the size of Japan) is moderately to severely degraded, and this percentage is increasing.

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The main cause of declining biological productivity in irrigated croplands is the accumulation of salts in the soil. There is an important difference between rainwater and the water used for dryland irrigation. Rainwater results from the condensation of water evaporated by sunlight. Essentially, rainwater is distilled seawater or lake water. In contrast, water used for irrigation is the result of runoff from precipitation. Runoff percolates through the soil, dissolving and collecting much of the salts it encounters, before finding its way into rivers or aquifers. When used to irrigate crops, runoff evaporates and leaves behind much of the salts that it collected. Irrigated crops need an average of 80 cm (about 30 inches) of water annually. These salts can build up in the soil unless additional water is used to flush them out. This process can rapidly transform productive land into relatively barren salt flats scattered with halophytes (plants adapted to high levels of salt in the soil).

Most salt-degraded cropland occurs in Asia and southwestern North America, which account for 75 and 15 percent of the worldwide total, respectively. In Asia, Iraq has lost over 70 percent of its irrigated land to salt accumulation. In Russia, much of the irrigated land located where the Volga River runs into the Caspian Sea may last only until the middle of the 21st century before the buildup of salts makes it virtually unusable. Such losses are not restricted to developing countries. In the United States, salt accumulation has lowered crop yields across more than 50,000 square km (19,000 square miles), an area that is about a quarter of the country’s irrigated land.