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Desertification
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Desertification is the degradation of land in arid and dry sub-humid areas, resulting primarily from natural activities and influenced by climatic variations. It is also a failure of the ecological succession process.
A major impact of desertification is biodiversity loss and loss of productive capacity, for example, by transition from land dominated by shrublands to non-native grasslands.
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Desertification is the degradation of land in arid and dry sub-humid areas, resulting primarily from natural activities and influenced by climatic variations. It is also a failure of the ecological succession process.
A major impact of desertification is biodiversity loss and loss of productive capacity, for example, by transition from land dominated by shrublands to non-native grasslands. In the semi-arid regions of southern California, many coastal sage scrub and chaparral ecosystems have been replaced by non-native, invasive grasses due to the shortening of fire return intervals. This can create a monoculture of annual grass that cannot support the wide range of animals once found in the original ecosystem. In Madagascar's central highland plateau, 10% of the entire country has been lost to desertification due to slash and burn agriculture by indigenous peoples. In Africa, if current trends of soil degradation continue, the continent will be able to feed only 25% of its population by 2025, according to UNU's Ghana-based Institute for Natural Resources in Africa. Globally, desertification claims a Nebraska-sized area of productive capacity each year.
Causes
Desertification is induced by several factors, primarily anthropogenic causes, which began in the Holocene era and continue at the highest pace today. The primary reasons for desertification are overgrazing, over-cultivation, increased fire frequency, water impoundment, deforestation, overdrafting of groundwater, increased soil salinity, and global climate change.
Deserts may be separated from surrounding, less arid areas by mountains and other contrasting landforms that reflect fundamental structural differences in the terrain. In other areas, desert fringes form a gradual transition from a dry to a more humid environment, making it more subtle to determine the desert border. These transition zones can have fragile, delicately balanced ecosystems. Desert fringes often are a mosaic of microclimates. Small hollows support vegetation that picks up heat from the hot winds and protects the land from the prevailing winds. After rainfall the vegetated areas are distinctly cooler than the surroundings.
In these marginal areas human activity may stress the ecosystem beyond its tolerance limit, resulting in degradation of the land. By pounding the soil with their hooves, livestock compact the substrate, increase the proportion of fine material, and reduce the percolation rate of the soil, thus encouraging erosion by wind and water. Grazing and collection of firewood reduce or eliminate plants that bind the soil and prevent erosion. All these come about due to the trend towards settling in one area instead of a nomadic culture.
Sand dunes can encroach on human habitats. Sand dunes move through a few different means, all of them assisted by wind. One way that dunes can move is through saltation, where sand particles skip along the ground like a rock thrown across a pond might skip across the water's surface. When these skipping particles land, they may knock into other particles and cause them to skip as well. With slightly stronger winds, particles collide in mid-air, causing sheet flows. In a major dust storm, dunes may move tens of meters through such sheet flows. And like snow, sand avalanches, falling down the steep slopes of the dunes that face away from the winds, also moving the dunes forward.
It is a common misconception that droughts by themselves cause desertification. While drought is a contributing factor, the root causes are all related to man's overexploitation of the environment.
There is no geological evidence that deserts expanded significantly before the advent of civilization. Droughts are common in arid and semiarid lands, and well-managed lands can recover from drought when the rains return. Continued land abuse during droughts, however, increases land degradation. Increased population and livestock pressure on marginal lands has accelerated desertification. In some areas, nomads moving to less arid areas disrupt the local ecosystem and increase the rate of erosion of the land. Nomads typically try to escape the desert, but because of their land-use practices, they are bringing the desert with them.
Some arid and semi-arid lands can support crops, but additional pressure from greater populations or decreases in rainfall can lead to the few plants present disappearing. The soil becomes exposed to wind, causing soil particles to be deposited elsewhere. The top layer becomes eroded. With the removal of shade, rates of evaporation increase and salts become drawn up to the surface. This increases soil salinity which inhibits plant growth. The loss of plants causes less moisture to be retained in the area, which may change the climate pattern leading to lower rainfall.
This degradation of formerly productive land is a complex process. It involves multiple causes, and it proceeds at varying rates in different climates. Desertification may intensify a general climatic trend toward greater aridity, or it may initiate a change in local climate. Desertification does not occur in linear, easily mappable patterns. Deserts advance erratically, forming patches on their borders. Areas far from natural deserts can degrade quickly to barren soil, rock, or sand through poor land management. The presence of a nearby desert has no direct relationship to desertification. Unfortunately, an area undergoing desertification is brought to public attention only after the process is well under way. Often little data are available to indicate the previous state of the ecosystem or the rate of degradation.
Combating desertification is complex and difficult, usually impossible without alteration of land management practises that led to the desertification. Over-exploitation of the land and climate variations can have identical impacts and be connected in feedbacks, which makes it very difficult to choose the right mitigation strategy. Investigating the historic desertification plays a special role since it allows better distinguishing of human and natural factors. In this context, recent research about historic desertification in Jordan questions the dominant role of man. It seems possible that current measures like reforestation projects cannot achieve their goals if global warming continues. Forests may die when it gets drier, and more frequent extreme events as testified in sediments from earlier periods could become a threat for agriculture, water supply, and infrastructure.
Prehistoric patterns
Desertification is a historic phenomenon; the world's great deserts were formed by natural processes interacting over long intervals of time. During most of these times, deserts have grown and shrunk independent of human activities. Paleodeserts are large sand seas now inactive because they are stabilized by vegetation, some extending beyond the present margins of core deserts, such as the Sahara. Many deserts in western Asia arose because of an overpopulation of prehistoric species and subspecies during the late Cretaceous era.
Dated fossil pollen indicates that today's Sahara desert has been changing between desert and fertile savanna. Studies also show that prehistorically the advance and retreat of deserts tracked yearly rainfall, whereas a pattern of increasing amounts of desert began with human-driven activities of overgrazing and deforestation.
A chief difference of prehistoric versus present desertification is the much greater rate of desertification than in prehistoric and geologic time scales, due to anthropogenic influences.
Historical and current desertification
Overgrazing and to a lesser extent drought in the 1930s transformed parts of the Great Plains in the United States into the "Dust Bowl". During that time, a considerable fraction of the plains population abandoned their homes to escape the unproductive lands. Improved agricultural and water management have prevented a disaster of the earlier magnitude from recurring, but desertification presently affects tens of millions of people with primary occurrence in the lesser developed countries.
Desertification is widespread in many areas of the People's Republic of China. The populations of rural areas have increased since 1949 for economical reasons as more people have settled there. While there has been an increase in livestock, the land available for grazing has decreased. Also the importing of European cattle such as Friesian and Simmental, which have higher food intakes, has made things worse.
Human overpopulation is leading to destruction of tropical wet forests and tropical dry forests, due to widening practices of slash-and-burn and other methods of subsistence farming necessitated by famines in lesser developed countries. A sequel to the deforestation is typically large scale erosion, loss of soil nutrients and sometimes total desertification. Examples of this extreme outcome can be seen on Madagascar's central highland plateau, where about seven percent of the country's total land mass has become barren, sterile land.
Overgrazing has made the Rio Puerco Basin of central New Mexico one of the most eroded river basins of the western United States and has increased the high sediment content of the river. Overgrazing is also contributing to desertification in some parts of Chile, Ethiopia, Morocco and other countries. Overgrazing is also an issue with some regions of South Africa such as the Waterberg Massif, although restoration of native habitat and game has been pursued vigorously since about 1980.
Another example of desertification occurring is in the Sahel. The chief cause of desertification in the Sahel is slash-and-burn farming practised by an expanding human population. The Sahara is expanding south at a rate of up to 48 kilometres per year.
The Desert of Maine is a dune of glacial silt near Freeport, Maine. Overgrazing and soil erosion exposed the cap of the dune, revealing the desert as a small patch that continued to grow, overtaking the land. The site's desert conditions are artificially maintained as a tourist attraction.
Ghana and Nigeria currently experience desertification; in the latter, desertification overtakes about of land per year. The Central Asian countries, Kazakhstan, Kyrgyzstan, Mongolia, Tajikistan, Turkmenistan, and Uzbekistan, are also affected. More than 80% of Afghanistan's and Pakistan's land could be subject to soil erosion and desertification. In Kazakhstan, nearly half of the cropland has been abandoned since 1980. In Iran, sand storms were said to have buried 124 villages in Sistan and Baluchestan Province in 2002, and they had to be abandoned. In Latin America, Mexico and Brazil are affected by desertification.
Countering desertification
Desertification has been recognized as a major threat to biodiversity. Numerous countries have developed Biodiversity Action Plans to counter its effects, particularly in relation to the protection of endangered flora and fauna.
A number of methods have been tried in order to reduce the rate of desertification and regain lost land; however, most measures treat symptoms of sand movement and do not address the root causes of land modification such as overgrazing, unsustainable farming (eg cattle farming) and deforestation by the indigenous population. In developing countries under threat of desertification, many local people use trees for firewood and cooking which has increased the problem of land degradation and often even increased their poverty. In order to gain further supplies of fuel the local population add more pressure to the depleted forests; adding to the desertification process.
Techniques focus on two aspects: provisioning of water (eg by wells and energy intesive systems involving water pipes over long distances) and fixating and hyper-fertilising soil.
Fixating the soil is often done through the use of shelter belts, woodlots and windbreaks. Windbreaks are made from trees and bushes and are used to reduce soil erosion and evapotranspiration. They were widely encouraged by development agencies from the middle of the 1980s in the Sahel area of Africa. Another approach is the spraying of petroleum or nano clay over semi-arid cropland. This is often done in areas where either petroleum or nano clay is easily and cheaply obtainable (eg Iran). In both cases, the application of the material coats seedlings to prevent moisture loss and stop them being blown away.
The enriching of the soil and the restoration of its fertility is often done by a variety of plants. Of these, the Leguminous plants which extracts nitrogen from the air and fixes it in the soil, and food crops/trees as grains, barley, beans and dates are the most important.
Finally, some approaches as stacking stones around the base of trees and artificial groove-digging also help in increasing the chance of success/crop survival. Stacked stones help to collect morning dew and retain soil moisture. Artificial grooves are dug in the ground as to retain rainfall and trap wind-blown seeds.
In order to solve the problem of chopping down trees for personal energy requirements, solutions as Solar ovens and efficient wood burning cook stoves are being advocated as a means to relieving some of this pressure upon the environment.
While desertification has received some publicity by the news media, most people are unaware of the extent of environmental degradation of productive lands and the expansion of deserts. In 1988 Ridley Nelson pointed out that desertification is a subtle and complex process of deterioration.
At the local level, individuals and governments can partially or temporarily forestall desertification. Sand fences are used throughout the Middle East and the US, in the same way snow fences are used in the north. Placement of straw grids, each up to a square meter in area, will also decrease the surface wind velocity. Shrubs and trees planted within the grids are protected by the straw until they take root. However, some studies suggest that planting of trees depletes water supplies in the area. In areas where some water is available for irrigation, shrubs planted on the lower one-third of a dune's windward side will stabilize the dune. This vegetation decreases the wind velocity near the base of the dune and prevents much of the sand from moving. Higher velocity winds at the top of the dune level it off and trees can be planted atop these flattened surfaces.
Oases and farmlands in windy regions are often protected by the approach described above by planting tree fences or grass belts in order to reduce erosion and walking dunes. Also, small projects as oases often section their plot of land by placing a barrier of thorny bushes or other obstacles to keep grazing animals of the food crops. Instead, they provide water provisioning (eg from a well, ...) outside this barrier. They provide this service mainly to accommodate the animals of travelers (eg camels, ...). Sand that manages to pass through the grass belts can be caught in strips of trees planted as wind breaks 50 to 100 meters apart adjacent to the belts. Small plots of trees may also be scattered inside oases to stabilize the area. On a much larger scale, a "Green Wall of China", which will eventually stretch more than 5,700 kilometers in length, nearly as long as the Great Wall of China, is being planted in north-eastern China to protect "sandy lands" – deserts created by human activity.
Africa, with coordination from Senegal, has launched its own "green wall" project. Trees will be planted on a 15 km wide land strip from Senegal to Djibouti. Aside from countering desert progression, the project is also aimed at creating new economic activities, especially thanks to tree products such as gum arabic
More efficient use of existing water resources and control of salinization are other tools for mitigating arid lands. New ways are also being sought to find groundwater resources and to develop more effective ways of irrigating arid and semiarid lands. Research on the reclamation of deserts is also focusing on discovering proper crop rotation to protect fragile soil, on understanding how sand-fixing plants can be adapted to local environments, and on how overgrazing can be addressed. A proposal combining desert stabilization and renewable energy is ADRECS - Aerially Delivered RE Afforestation and Environment Control -
Mitigation concepts
A recent development is the Seawater Greenhouse and Seawater Forest. This proposal is to construct these devices on coastal deserts in order to create freshwater and grow food
A similar approach is the Desert Rose concept
These approaches are of unproven widespread applicability, since sites must be chosen that are below sea level.
See also
Further reading
- Benjaminsen, Tor A., and Gunvor Berge (2000). Timbuktu: myter, menneske, miljř. Oslo: Spartakus forlag
- Lucke, Bernhard (2007): Demise of the Decapolis. Past and Present Desertification in the Context of Soil Development, Land Use, and Climate. Online at
- by Lester R. Brown
- Geist, Helmut (2005) The Causes and Progression of Desertification, Abingdon: Ashgate
- Millennium Ecosystem Assessment (2005)
- Reynolds, James F., and D. Mark Stafford Smith (ed.) (2002) Global Desertification – Do Humans Cause Deserts? Dahlem Workshop Report 88, Berlin: Dahlem University Press
- Stock, Robert (1995). Africa South of the Sahara. New York: The Guilford Press
External links
- - A global agricultural research-for-development partnership against desertification
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- in Nevada, United States
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- [TEMA]
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- - A key player in combating desertification and drought effects
- UNEP (2006):
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- United Nations Convention to Combat Desertification -
- on the
- by David A. Bainbridge
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News
- Report on a project to stop the advance of the Sahara in Algeria - IPS, 27 February 2007
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