About one billion people live in arid or semiarid desert lands that occupy about one third of the world's land surface. In these drier parts of the world, deserts are increasing rapidly from a combination of natural processes and human activities, a process known as desertification or land degradation. An annual rainfall of less than 10 in (25 cm) will produce a desert anywhere in the world. In the semiarid areas along the desert margins, where the annual rainfall is around 16 in (40 cm), the ecosystem is inherently fragile with seasonal rains supporting the temporary growth of plants. Recent changes in the climate of these regions have meant that the rains are now unreliable and the lands that were once semiarid are now becoming desert. The process of desertification is precipitated by prolonged droughts, causing the top layers of the soil to dry out and blow away. The eroded soils become unstable and compacted and do not readily allow for seeding. This means that desertified areas do not regenerate by themselves but remain bare and continue to erode. Desertification of grazing lands or croplands is accompanied, therefore, by a sharp drop in the productivity of the land.
Natural desertification is greatly accelerated by human activities that leave soils vulnerable to erosion by wind and water. The drier grasslands with too little rain to support cultivated crops have traditionally been used for grazing livestock. When semiarid land is overgrazed (by keeping too many animals on too little land), plants that could survive moderate grazing are uprooted and destroyed altogether. Since plant roots no longer bind the soil together, the exposed soil dries out and is blown away as dust. The destruction and removal of the topsoil means that soil productivity drops drastically. The obvious solution to desertification caused by overgrazing is to limit grazing to what the land can sustain, a concept that is easy to espouse but difficult to practice.
In the Sahel zone along the southern edge of the Sahara desert, settled agriculture and overgrazing livestock on the fragile scrublands have led to widespread soil erosion. Nomadic pastoralists, who have traditionally followed their herds and flocks in search of new pastures, are now prevented by national borders from reaching their chosen grazing grounds. Instead of migrating, the nomads have been encouraged to settle permanently and this has led to their herds overgrazing available pastures.
Other human factors leading to desertification include over-cultivation, deforestation, salting of the soil through irrigation, and the plowing of marginal land. These destructive practices are intensified in developing countries by rapid population growth, high population density, poverty, and poor land management. The consequences of desertification in some countries mean intensified drought and famine and lowered standards of living. It is estimated that desertification worldwide has claimed an area the size of Brazil (2 billion acres or 810 million ha) in the past 50 years. Each year new deserts consume an area the size of Belgium (15 million acres or 6 million ha), most of which is in the African Sahel.
In marginal areas throughout the world, traditional farming practices can lead to desertification. Plowing turns the top layer of the soil upside down, burying and killing weeds but exposing bare soil to erosion. In arid areas theexposed soil dries out rapidly and is easily lost through wind erosion.
World deserts and areas at risk of desertification. (Beacon Press. Reproduced by permission.)
The processes of erosion and soil formation vary with climate and with the composition of the parent material. In balanced ecosystems, soil lost to erosion is replaced by new soil created by natural processes. On average, new soil is formed at a rate of about 5 tons an acre (12.5 tons per ha) per year, which is equivalent to a layer of soil about 0.2 in (0.4 cm) thick. This means that soils can sustain an erosion rate of up to 5 tons per acre per year and still remain in balance. However, much of the world's crop and forest land is not within this balance (with erosion running at two to 10 times the tolerable rate). In the United States about 6 tons of topsoil are lost for every ton of grains produced.
Forests are cut down for many different reasons. Some are cleared for agriculture, some for construction, some for paper products, and some to meet cooking and heating needs. Unfortunately, deforestation results in more than just the loss of trees, for soil is eroded, nutrients are lost to the ecosystem, and the water cycle is disrupted. The roots of the trees serve to bind the soil together and to hold water in the ecosystem, while the leaves of the trees break the force of the rain and allow it to soak into the topsoil. The result is that surface runoff from a forested hillside is half as much as from a grass-covered slope. Additionally, water soaking into the ground (rather than running off) leads to the natural recharge of groundwater and other water resources. Water and soil runoff from deforested hillsides cause flooding and siltation of agricultural and aquatic ecosystems in adjacent lowlands. Forests are also more efficient at reabsorbing and recycling the nutrients released from decaying detritus than are grasslands. Clearing forests therefore exposes the soil to both erosion and nutrient loss and alters the recharge of water reserves in the ecosystem.
Industrialized countries experienced a period of intense deforestation during the Industrial Revolution and even today much of the land has low productivity. Fortunately, most of these countries are now reforesting faster than they are deforesting. This is possible because their population growth is low, their agricultural production per acre is high, and their need for fuelwood for cooking and heating is optional, since fossil fuels and electricity are widely available. All of these factors release the pressure to further deforest the land.
In developing countries, on the other hand, high population growth rates and widespread poverty put pressure on the forests. Trees are needed for firewood, charcoal, and export, and the land is needed for farmland. In some developing countries deforestation exceeds replanting by five times. In Ethiopia, population and economic pressures pushed people to deforest and cultivate hillsides and marginally dry lands, and more than 1 billion tons of topsoil per year are now lost, resulting in recurrent famines. In parts of India and Africa there is so little wood available that dried animal dung is used to fuel cooking fires, an act that further robs the soil of potential nutrients. In Brazil, soil erosion and desertification have resulted from the conversion of forests to cattle ranches. In China, about one-third of its agricultural land has been lost to erosion, and the story is similar for many other countries.
The answer to erosion from deforestation is reforestation, better forest conservation, and better forest management to increase productivity. Planting trees on hillsides is particularly effective. Reforesting desertified areas first requires mulching the soil to hold moisture and the protection of the seedlings for several years until natural processes can regenerate the soil. Using these methods, Israel has achieved spectacular success in bringing desertlands (a product of past desertification) back to agriculture.
Desertification and its agents, deforestation and erosion, have been powerful shapers of human history. Agriculture had its roots in the once fertile crescent of the Middle East and in the Mediterranean lands. However, deforestation, overgrazing, and poor agricultural practices have turned once-productive pastureland and farmland into the near deserts of today. It is thought by some that deforestation and desertification may have even contributed to the collapse of the Greek and Roman Empires. Similar fates may have befallen the Harappan civilization in India's Indus Valley, the Mayan civilization in Central America, and the Anasazi civilization of Chaco Canyon, New Mexico, in what is today desertland.
Resources
Books
Dejene, A. Environment, Famine, and Politics in Ethiopia: A View from the Village. Boulder, CO: L. Rienner Publishers, 1990.
Grainger, A. Desertification: How People Make Deserts, How People Can Stop, and Why They Don't. London: International Institute for Environment and Development, 1982.
McLeish, E. The Spread of Deserts. Austin, TX: Steck-Vaughn Library, 1991.
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