Land degradation in low-rainfall and seasonally dry areas of the Earth. It can be viewed as both a process and the resulting condition. Desertification involves the impoverishment of vegetation and soil resources. Key characteristics include the degradation of natural vegetation cover and undesirable changes in the composition of forage species, deterioration in soil quality, decreasing water availability, and increased soil erosion from wind and water. Desertification is a global problem. Various stages of desertification can be seen in most of the world's drylands. In rare cases, desertification leads to abandoned, desertlike landscapes.
Causes and consequences
Although some authorities believe that climate change may be a causal factor, it is generally agreed that human activities, particularly excessive resource use and abusive land-use practices, are the primary cause of desertification. Specific activities leading to desertification include clearing and cultivation of low-rainfall areas where such cultivation is not sustainable, overgrazing of rangelands, clearing of woody plant species for fuelwood and building materials, and mismanagement of irrigated cropland leading to the buildup of mineral salts in the soil (salinization). Drought is often cited as a basic cause of desertification; however, it merely accelerates or accentuates land degradation processes already under way. See also: Drought
Consequences of desertification include reduced biological productivity, reduction of biodiversity, a gradual loss of agricultural potential and resource value, loss of food security, reduced carrying capacity for humans and livestock, increased risks from drought and flooding, and in extreme cases, barren lands that are effectively beyond restoration. Paleostudies, supported by model simulations, have shown that the intensity of Northern Hemisphere desert conditions has waxed and waned over the past 9000 years in response to the precession of the Earth's orbit about the Sun. Thus, it may be that the causal factors of desertification, whether climate change or human activities, depend on the time scale being addressed.
Concept
Desertification is an age-old problem in the world's drylands. For example, substantial areas in the region formerly known as Mesopotamia (the lower Tigris and Euphrates river basins of modern-day Iraq) have never recovered from desertification processes occurring over 4000 years ago. Likewise, much of the Mediterranean littoral has experienced varying degrees of desertification from excessive resource use beginning in the Roman era. Elsewhere in the world, on a lesser scale, desertification is known to have occurred centuries ago.
Beginning in the 1920s, there were warnings of the expansion or encroachment of the Sahara Desert into agrarian areas in the Sahel to the south. Massive soil erosion in the dust bowl in the United States during the 1930s pointed to the dangers of land degradation in drylands from unwise resource use.
The term "desertification" was coined in 1949 by A. Aubreville, a French forester, who described the creation of desertlike conditions resulting from deforestation in the seasonally dry subhumid Sudanian belt south of the Sahel in Africa. It was not until the 1970s, however, that the term came into widespread international use. The reason was the drought in the African Sahel which began in 1968 and which by 1973 had resulted in at least 100,000 deaths and many times more refugees. It became obvious that the famine was caused by more than drought: an environmental calamity had been developing for many years due to unsustainable patterns of stockraising and cultivation that were incompatible with the region's environmental limitations. Drought served as a catalyst to reveal the damage caused by overgrazing and overcultivation. The resulting desertlike conditions made it seem that the Sahara was expanding. Similar land degradation was occurring in many other drylands, arousing fears of climate change and the specter of a global crisis.
As a result of these events, the United Nations (UN) convened the first international conference on desertification (UNCOD) in 1977. The conference aspired to provide a comprehensive understanding of desertification and its consequences for development, as well as to develop a plan of action to combat this global threat in its various settings. In 1978 the United Nations Environment Programme (UNEP) estimated that over 1.15 107 mi2 (3 107 km2) were severely desertified. In 1984 UNEP estimated that 35% of the world's land surface was at risk and that nearly 5.2 107 acres (2.1 107 hectares) of productive land were being reduced to near or complete uselessness each year. This agency actively promoted the theory that the Sahara Desert was expanding southward at an average of almost 3.7 mi (6 km) per year. In response, a number of "green belt" schemes were implemented to try to halt the desert's expansion with rows of trees. In 1991 UNEP estimated that 73% of the world's rangeland, 47% of its rain-fed cropland, and 30% of its irrigated cropland was at least moderately desertified. This agency also asserted that the area irreversibly lost to desert was equivalent to all of the Earth's presently cultivated land and that desertification jeopardized the livelihoods of over a billion people in more than 100 countries. As a result of such claims, desertification became regarded as one of the world's most pressing environmental problems. During the 1992 UN "Earth Summit" Conference on Environment and Development in Rio de Janeiro, world leaders agreed to initiate negotiations on an international convention to address global desertification. The resulting Convention to Combat Desertification (CCD) was adopted in 1994. Following ratification by 50 countries, it became international law at the end of 1996. The convention's objective is to combat desertification and mitigate the effects of drought, particularly in Africa and other developing regions.
Controversies
Unfortunately, no universally agreed-upon definition of desertification emerged from the 1977 UNCOD forum, and by the early 1980s there were more than a hundred different definitions of this phenomenon describing land degradation in ecosystems ranging from near-deserts to moist tropical rainforests. The lack of precision in defining the concept of desertification generated a series of controversies which gradually have called the validity of the term, and the concept itself, into question. It is argued that the term "desertification" should be avoided altogether in preference for the more general term "land degradation." The latter is preferred because it can be applied to all terrestrial ecosystems and avoids the misconceptions evoked by the term "desertification." There is also a basic disagreement over which ecosystems should be included in the purview of desertification researchdrylands only, or all ecosystems in which degradation leads to drier conditions.
The struggle to define and determine the scope of desertification has become politicized. Primarily for political reasonsto focus international assistance efforts on poorer drought-prone developing countries, especially in the African Sahelworld leaders at the 1992 "Earth Summit" agreed to define desertification as "land degradation in arid, semi-arid, and dry subhumid areas resulting from various factors, including climatic variations and human activities." This definition was adopted by the CCD. However, the term "desertification" continues to be commonly applied in humid biomes as wellfor example, for the degradation of tropical rainforest into savanna grassland.
There is also a fundamental controversy over the global extent of desertification. Most existing definitions of desertification are too general to be truly useful for estimating the extent or severity of this phenomenon. Global estimates are based on informed opinion rather than field research or remotely sensed data, and the extent of the problem may be grossly overstated. In the case of the Sahara Desert, the official wisdom during the late 1970s and 1980s was that the desert was expanding southward at a substantial rate of 3.4-5.6 mi (5.5-9 km) per year, but critics charged that this claim was based on a flawed 1975 study. Subsequent studies based on satellite image interpretation have conclusively demonstrated that there has been no permanent shift of the Sahara's southern boundary. These studies show that the Sahara boundary shifts from year to year in response to fluctuations in precipitation. According to these studies, rainfall fluctuations also adequately explain all variations in vegetative land cover from year to year. These studies seriously challenge the assertions by UNEP and other organizations that there has been a massive loss of land productivity in the Sahel from desertification. They also challenge estimates that tens of thousands of square miles of productive land are being lost to desertification each year.
There is a fundamental question about whether desertificationin the sense of extensive new desertlike environments being created through human activities and possibly climate changeis even occurring. By the mid-1990s, a revisionist critique of the conventional concept of desertification was firmly established. Key elements of this critique include the following: (1) Much of the data supporting the conventional concept of desertification was flawed, because it was collected during drought periods. (2) The concept relied too heavily on premature assumptions (for example, desert advance) and minimal field data. (3) It is exceedingly difficult to distinguish between the temporary effects of drought and the more permanent condition of desertification. This difficulty raises serious doubts about the validity of many traditional desertification studies and most estimates of the global extent of desertification. (4) Much of the literature on desertification ignores the fact that most of the world's drylands and desert areas are natural ecosystems. For example, the 35% of the Earth's land surface that is commonly claimed to be at risk of desertification includes the Sahara Desert and the world's other natural desertsareas that are hardly at risk. (5) The traditional vision of desertification as the spread of desertlike conditions fails to recognize that land degradation in dryland areas often involves less obvious processes such as the gradual replacement of desirable forage plants by unpalatable species, with no net loss of primary productivity. (6) The imprecise and often-conflicting definitions of desertification have prevented a precise understanding of the actual problem and therefore have prevented effective treatment.
By the late-1990s, a major paradigm shift in drylands research was well established. Most specialists now acknowledge the difficulty of distinguishing between drought effects and human-induced degradation. Researchers increasingly focus on the regenerative capacity of dryland areas or on the complex localized interactions among climate variability, human activities, and the land surface. Few specialists would deny that extensive degradation of vegetation and soil resources is occurring in many low-rainfall and seasonally dry environments. However, they increasingly concur that these are variations of land degradation processes also occurring in more humid environments, rather than a unique problem called desertification.
Will Swearingen
http://www.AccessScience.com
Copyright McGraw-Hill
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