Desertification (Encyclopedia of Environmental Issues, Revised Edition)
Deserts are climatic regions that receive less than 25 centimeters (10 inches) of precipitation per year. They constitute the most widespread of all climates of the world and occupy 25 percent of the world’s land area. Most deserts are surrounded by semiarid climates referred to as steppes, which occupy 8 percent of the world’s lands. Deserts occur in the interiors of continents, on the leeward sides of mountains, and along the west sides of continents in subtropical regions. All of the world’s deserts risk further desertification.
Scientists use various methods to determine the historical climatic conditions of a region. These methods include studies of the historical distribution of trees and shrubs as determined from deposit patterns in lakes and bogs, patterns of ancient sand dunes, changes in lake levels through time, archaeological records, and tree rings (dendrochronology).
The largest deserts occur in North Africa, Asia, Australia, and North America. Four thousand to six thousand years ago, these desert areas were less extensive and were occupied by prairie or savanna grasslands. Rock paintings found in the Sahara show that humans during an earlier era in that region hunted buffalo and raised cattle on grasslands where giraffes browsed. The region near the Tigris and Euphrates rivers was also fertile. In the desert of northwest India, cattle and goats were grazed, and people lived in cities that have long since been...
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Causes (Encyclopedia of Environmental Issues, Revised Edition)
Two main factors influence the process of desertification: climatic variations and human activities. The major deserts of the world are located in areas of high atmospheric pressure, which experience subsiding dry air unfavorable to precipitation. Since the late 1960’s subtropical deserts have been experiencing prolonged periods of drought that have caused these areas to be dryer than usual.
The problem of desertification came to the attention of the world during the late 1960’s and early 1970’s as a result of severe drought in the Sahel, a region that extends in an east-west direction along the southern margin of the Sahara in West Africa. Rainfall declined an average of 30 percent in the Sahel, and scientific research was conducted to examine the natural mechanisms causing the drought. One set of studies was related to changes in global circulation patterns associated with changes in heat distribution in the oceans. A correlation was found between sea surface temperatures and the reduction of rainfall in the Sahel. It was determined that the Atlantic Ocean’s higher surface temperatures south of the equator and lower temperatures north of the equator west of Africa are associated with lower precipitation in northern tropical Africa. However, the exact cause of the change in sea surface temperature patterns remained undetermined.
Land-cover changes play a major role in desertification. A lack of rain causes the ground...
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Consequences (Encyclopedia of Environmental Issues, Revised Edition)
A reduction in vegetation cover and soil quality may affect the local climate by causing a rise in temperatures and a reduction in moisture. This can, in turn, have impacts on the area beyond the desert by causing changes in the climate and atmospheric patterns of the region. Substantial vegetation cover changes in humid and subhumid areas have the potential to cause significant regional climatic changes. Desertification is a global problem because it can cause the loss of vegetation and animal diversity, as well as the pollution of rivers, lakes, and oceans. As a result of excessive rainfall and flooding in subhumid areas, fields lacking sufficient vegetation may be eroded by runoff.
Desertification can also lead to food shortages, which are often accompanied by social unrest. Unless food production, distribution, and costs can meet the needs of the world’s expanding population, hunger and other untenable conditions associated with desertification could cause millions of refugees and emigrants to flee the developing and the least developed countries. The ongoing loss of productive land to desertification, however, affects the world’s ability to keep up with global requirements for food production.
Studies have been conducted to determine how rising levels of greenhouse gases are likely to affect the rate of desertification. Rising temperatures and changes in rainfall patterns influence desertification;...
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Combating Desertification (Encyclopedia of Environmental Issues, Revised Edition)
In response to the 1968-1974 drought in Sahelian West Africa, representatives from various countries met in Nairobi, Kenya, in 1977 for a United Nations conference on desertification. The conference resulted in the Plan of Action to Combat Desertification, which listed twenty-eight measures that national, regional, and international organizations could take to combat land degradation. A lack of adequate funding and commitment by governments caused the plan to fail, however; when the United Nations Environment Programme (UNEP) assessed the plan in 1991, it found that little had been accomplished and that the desertification problem had worsened.
As a result of the 1977 United Nations conference, several countries developed national plans of action to combat desertification. In Kenya, local organizations worked with primary schools to plant five thousand to ten thousand seedlings per year. One U.S.-based organization promoted reforestation by providing materials to establish nurseries, training programs, and extension services. Community-level efforts to combat desertification yielded promising results, and UNEP recognized that such projects tend to have a greater success rate than top-down projects. Participants at the Earth Summit, held in Rio de Janeiro, Brazil, in 1992, supported the concept of sustainable development at the community level to combat the problem of desertification.
In 1994, the United...
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Further Reading (Encyclopedia of Environmental Issues, Revised Edition)
Chiras, Daniel D. “Creating a Sustainable System of Agriculture to Feed the World’s People.” In Environmental Science. 8th ed. Sudbury, Mass.: Jones and Bartlett, 2010.
Geist, Helmut. The Causes and Progression of Desertification. Burlington, Vt.: Ashgate, 2005.
Glantz, Michael H., ed. Desertification: Environmental Degradation in and Around Arid Lands. Boulder, Colo.: Westview Press, 1977.
Gore, Rick. “The Desert: An Age-Old Challenge Grows.” National Geographic, November, 1979, 594-639.
Goudie, Andrew. “The Human Impact on Vegetation.” In The Human Impact on the Natural Environment: Past, Present, and Future. 6th ed. Malden, Mass.: Blackwell, 2005.
Hulme, Mike, and Mick Kelly. “Exploring the Links Between Desertification and Climate Change.” Environment 35, no. 6 (July/August, 1993): 4-1.
United Nations. Convention to Combat Desertification Secretariat. Desertification: Coping with Today’s Global Challenges. Eschborn, Germany: Author, 2008.
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Background (Encyclopedia of Global Resources)
Desertification occurs over a period of years, with numerous factors interacting to stress the environment. Desertification results when social, political, and economic forces cause agricultural exploitation beyond the carrying capacity of marginal arid lands. Airborne and waterborne erosion strip vulnerable topsoil, rendering the land less productive over time. People living in areas undergoing desertification have poor harvests and are increasingly unable to feed themselves.
Although desertification can result from natural causes over eons, it can occur in relatively short periods of time, notably as a result of improper land-use management—usually a combination of deforestation and overgrazing on semiarid grasslands. Nonnative cash crops and monoculture plantations quickly deplete soils, impoverishing the ecosystem and accelerating desertification. In developed countries, boom and bust agricultural practices in arid lands cause overexpansion during favorable climatic conditions, followed by drought. People living on marginal lands in less developed countries depend on subsistence agricultural production. Drought causes these people severe hardships, as their crops fail and animal herds die. Sustainable development of semiarid lands in less developed countries would benefit many impoverished people worldwide.
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Desertification in the Western Hemisphere (Encyclopedia of Global Resources)
In 1806, Zebulon Pike characterized the southern plains as a sandy wasteland, an area later called the “Great American Desert” on maps. Following the Homestead Act of 1862, 65 hectares of shortgrass could be claimed by settlers living and working on the land, and some people thought that to be sufficient incentive to migrate there. Amish and Mennonite settlers found the 65 hectares sufficient for their type of agriculture, but the concept of large commercial ranches and farms, coupled with the introduction of machinery, prompted the passage of the Enlarged Homestead Act of 1909, resulting in a land rush.
By the time World War I began, wheat was the favored crop in the Great Plains. Previously uncultivated land throughout Kansas, Colorado, New Mexico, Oklahoma, and Texas was plowed and planted with wheat by residents and absentee farmer-speculators between the end of World War I and 1930. When a lengthy drought began in 1931, precipitation was spotty, and wheat crops began failing. Fields were abandoned, and the airborne soil erosion which characterized the Dust Bowl began.
Within the United States, elimination of homesteading, massive purchase of marginal land, large public works projects, and agricultural subsidies helped reclaim marginal desert lands throughout the Great Plains and Southwest. The “New Deal” of President Franklin Delano Roosevelt began land reclamation using the Soil...
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The Sahel: Southern Encroachment of the Saharan Desert (Encyclopedia of Global Resources)
The African region known as the Sahel, which roughly follows the 15° north parallel, is an example of dry woodland and dry wooded grassland undergoing desertification. Its northern border with the Sahara Desert receives 150 millimeters of annual rainfall; its southern border receives 600 millimeters of rain yearly.
Between 1931 and 1960 the Sahel experienced greater than normal levels of precipitation, coupled with a doubling in population. During this time, a large part of the Sahel was populated by nomadic peoples herding small livestock flocks and practicing subsistence agriculture. In 1970, a period of decades-long drought began. This drought signaled a period of overgrazing by livestock and deforestation to feed cooking fires, two key factors in accelerating desertification. Starting in 1968, the Sahel began to move southward from its 1931-1960 boundaries. Warmer ocean waters changed rainfall patterns, resulting in less precipitation in the northern Sahel in the late twentieth century. More than 60 percent of foreign aid in the region was expended on road construction, which consumed valuable water resources. Development of large-scale dams in the Senegal River Valley during the 1980’s caused population displacement because much of the irrigated land was taken over by large plantations growing cash crops of peanuts, rice, and cotton. Deforestation to expand plantations was...
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Mitigation of Desertification (Encyclopedia of Global Resources)
Identification of the problems caused by desertification by national governments and international agencies led to concerted efforts to halt and reverse the process during the late twentieth and early twenty-first centuries. Developed nations, including the United States and Australia, have relied heavily on technology to combat desertification. Ranchers have formed effective organizations to lobby the central governments for aid. Drilling wells, building dams, installing irrigation canals, using chemical fertilizers on depleted soil, using crop-dusting airplanes to limit pest damage, and implementing large-scale reforestation and revegetation programs are some of the methods used to combat desertification.
In Australia, ranchers of large areas import feed in drought years and move herds of cattle and sheep out of desert environments to more productive grazing land. Government-sponsored selective breeding programs produced special cattle breeds—the Belmont Red and the Australian Milking Zebu—better suited to production in arid lands.
Less developed countries rarely have the resources to purchase technology. International aid efforts focused on building dams have been found to result in dislocation of hundreds of thousands of people while worsening public health problems like schistosomiasis and malaria. Because of these problems, beginning in the 1990’s, international aid organizations increasingly...
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Further Reading (Encyclopedia of Global Resources)
Hazell, Peter, and Stanley Wood. “Drivers of Change in Global Agriculture.” Philosophical Transactions of the Royal Society of London 363, no. 1491 (February 12, 2008): 495-515.
Liebesman, Lawrence R., and Rafe Petersen. Endangered Species Deskbook. Washington, D.C.: Environmental Law Institute, 2003.
Worster, Donald. Dust Bowl: The Southern Plains in the 1930’s. 25th anniversary ed. New York: Oxford University Press, 2004.
Environment Canada: Canadian Wildlife Service. Species at Risk. http://www.cws-scf.ec.gc.ca/theme.cfm?lang=e&category=12
NOAA Fisheries Service: Office of Protected Resources. Endangered Species Act. http://www.nmfs.noaa.gov/pr/laws/esa
U.S. Fish and Wildlife Service. Endangered Species Program. http://www.fws.gov/endangered
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Background (Encyclopedia of Global Warming)
Desertification is a process of land degradation in arid and semiarid areas resulting from climatic variations and human activities. Degradation results from pressure from expansion of agriculture and livestock numbers, which make the land increasingly vulnerable to the impact of drought. Also, rising human populations have led people to farm on increasingly marginal land, which is even more at risk. The pressure on the land is such that, when drought occurs, the land degrades to the point that it is unable fully to recover.
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Characteristics of Desertification (Encyclopedia of Global Warming)
Manifestations of desertification include a breakdown of soil structure, accelerated soil loss to wind and water erosion, an increase in atmospheric dust, a reduction in soil moisture-holding capacity, an increase in surface-water runoff and streamflow variability, salinization of soils and groundwater, and reductions in species diversity and plant biomass. The net result is a reduction in the overall productivity of dry-land ecosystems. This reduction leads to the impoverishment of human communities that are dependent on the land for survival.
The best examples of desertification are to be found in the Sahel region of Africa and the Rajasthan state of India, vulnerable areas on the borders of the Sahara and Thar Deserts, respectively. The underlying problem is poverty, which means few resources are available for managing the environment. Most of the people are subsistence farmers whose food supply is dependent on an adequate harvest each year. Farmers rely on summer monsoon rains. If one rainy season fails, people have very little in the way of stored food or money to see them through. The most vulnerable are the pastoralists, whose animals rapidly weaken and perish when there is nothing left to graze. Those animals that do survive will have stripped the land of vegetation so intensively that it may fail fully to recover when the rains eventually return.
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Climatic Feedback Processes (Encyclopedia of Global Warming)
The impact of drought is in some cases linked to feedback processes between the atmosphere and a land surface that is modified and used by the very population that is at risk. Stress on the land is not the sole cause of desertification, but it weakens an ecosystem’s ability to withstand drought. It is an important part of the feedback chain. In wet years, there is often an expansion and intensification of grazing and cultivation of land that is otherwise marginal. Following a relatively dry year, excessive demands may be placed on the water stored in the soil. The soil will then dry, become susceptible to wind erosion, and eventually blow away. Even if the rains were to return, what soil remains would be washed away by sheet erosion and gullying. Most important, if such changes affect a large area, positive feedback processes to which localized climate is highly sensitive are set in motion, which accentuates existing anomalies in climate.
A key factor in the role of climate is stability of air. When the atmosphere is stable, upward motion of air is suppressed. Even in humid airstreams, rainfall will not occur unless stability is overcome. Subtropical high-pressure belts at about 30° latitude on either side of the equator are associated with masses of stable air. This air accounts for most of Earth’s large areas of arid and semiarid climate. Rain occurs when the air in these high-pressure belts is displaced by...
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Desertification and Climate Change (Encyclopedia of Global Warming)
The Intergovernmental Panel on Climate Change (IPCC) has commented on the possibility of increased frequency of droughts in certain areas. If the vulnerable desert borderlands are among the regions affected, then desertification may be intensified. However, connections between anthropogenic increases in greenhouse gases (GHGs) in the atmosphere and changed drought frequency and intensity are only speculative, as climate models that are used to assess these connections have not been shown to be reliable. The level of scientific uncertainty and the existence of conflicting results are such that reliable predictions of future climate are not possible at this time.
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Context (Encyclopedia of Global Warming)
Owing to the uncertainty surrounding scientists’ understanding of the global climate, neither the trends in drought occurrence nor the interannual variability of droughts can be simulated reliably in global climate models. Despite this, projections have been made about future trends in precipitation extremes linked to increases in GHGs. Vulnerability will decline if drought frequency and intensity are reduced. The salient point is that there is no clear answer to the question of what will happen to trends in drought occurrence.
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Further Reading (Encyclopedia of Global Warming)
Delville, Philippe L. Societies and Nature in the Sahel. London: Taylor & Francis, 2007. Explores the links between environmental and social systems that lead to desertification across sub-Saharan Africa.
Geist, Helmut. The Causes and Progression of Desertification. Burlington, Vt.: Ashgate, 2005. Examines desertification on the local and international scales and assesses the role of causal processes.
Middleton, N. Global Desertification: Do Humans Cause Deserts? Amsterdam, the Netherlands: Elsevier, 2004. Explores and assesses various possible causes of desertification.
Williams, M. A. J., and Robert C. Balling, Jr. Interactions of Desertification and Climate. London: Arnold, 1996. Comprehensive account of the interactions between desertification and climate.
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