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Experts disagree over the current rate at which arable land is becoming desert, to what extent human activities are responsible, and whether the process is reversible. Yet events such as the Dust Bowl of the 1930s in the United States are compelling evidence of a link between desertification and human misuse of the land.

Desertification is the process of land becoming desert, as from human mismanagement or climate change. It remains a controversial issue with regard to definition, nature, rate of spread, irreversibility, and causation. Nonetheless, it is a serious example of land degradation in dry lands. Deserts have repeatedly expanded and contracted during the last few millions of years in response to climate changes, but their margins are now being affected by a suite of increasing human pressures that depletes soil and vegetation resources.

Desertification was first used as a term, but not formally defined, by a French forester named “Aubreville” in 1949, and for some years the term desertization was defined as “the spread of desert-like conditions in arid or semi-arid areas, due to man’s influence or to climatic change” (Rapp 1974, 3).

Different experts have defined desertification according to its cause. Some definitions stress the importance of anthropogenic (human-caused) actions. The scientist Harold E. Dregne says, “Desertification is the impoverishment of terrestrial ecosystems under the impact of man. It is the process of deterioration in these ecosystems that can be measured by reduced productivity of desirable plants, undesirable alterations in the biomass [the amount of living matter] and the diversity of the micro and macro fauna and flora, accelerated soil deterioration, and increased hazards for human occupancy” (Dregne 1986, 6–7).

Other studies acknowledge the possible importance of natural climatic controls but give them a relatively lesser role. In a report for the U.S. Department of the Interior / Bureau of Land Management, desertification is defined as “the sustained decline and/or destruction of the biological productivity of arid and semi arid lands caused by manmade stresses, sometimes in conjunction with natural extreme events. Such stresses, if continued or unchecked, over the long term may lead to ecological degradation and ultimately to desert-like conditions” (Sabadell et al. 1982, 7).

Other experts are more even-handed with respect to the balance of anthropogenic and natural causes: “A simple and graphic meaning of the word ‘desertification’ is the development of desert like landscapes in areas which were once green. Its practical meaning . . . is a sustained decline in the yield of useful crops from a dry area accompanying certain kinds of environmental change, both natural and induced” (Warren and Maizels 1976, 1).

Experts are not sure how extensive desertification is or how fast it is progressing. The lack of agreement on the former process makes determining the latter difficult, prompting some to remark, “Desertification will remain an ephemeral concept to many people until better estimates of its extent and rate of increase can be made on the basis of actual measurements” (Grainger 1990, 145).

The United Nations Environment Programme (UNEP) has played a pivotal role in the promotion of desertification as an environmental issue, as is made evident by this statement from The World Environment 1972–1992: “Desertification is the main environmental problem of arid lands, which occupy more than 40 per cent of the total global land area. At present, desertification threatens about 3.6 billion hectares—70 percent of potentially dry lands, or nearly one-quarter of the total land area of the world. These figures exclude natural hyper-arid deserts. About one sixth of the world’s population is affected” (Tolba and El-Kholy 1992, 134).

Some scholars, however, have been critical of UNEP’s views on the amount of land that is desertified. They state: “The bases for such data are at best inaccurate and at worst centered on nothing better than guesswork. The advancing desert concept may have been useful as a publicity tool but it is not one that represents the real nature of desertification processes” (Thomas and Middleton 1994, 160).

Despite its critics, UNEP continues to support issues of desertification. UNEP helped established the U.N. Convention to Combat Desertification (UNCCD), which holds an international World Day to Combat Desertification and Drought on 17 June. These worldwide annual events supported by UNEP and the UNCCD seek to raise awareness and promotion of desertification issues. In 2006, UNEP focused the international World Environment Day (held annually on 5 June) on desertification issues with the theme “Don’t Desert Drylands.”

Rates of Desertification

Experts have conducted relatively few reliable studies of the rate of desert advance. In 1975, the British ecologist Hugh Lamprey attempted to measure the shift of vegetation zones in the Sudan in Africa and concluded that a portion of the Sahara Desert in Sudan had advanced by 90 to 100 kilometers between 1958 and 1975, with an average rate of about 5.5 kilometers per year. On the basis of analysis of remotely sensed data and ground observations, however, other scholars found limited evidence that this ad-vance had taken place (Hellden 1984). One problem is that substantial fluctuations in vegetation production may take place from year to year. Meteorological satellite observations of green biomass production levels on the southern margins of the Sahara Desert have revealed such fluctuations.

The International Soil Reference Center in the Netherlands conducted a global assessment by soil degradation on behalf of UNEP during the late 1980s and early 1990s. The center used a Geographical Information System to analyze data collected through a clearly defined, but largely qualitative, methodology. Despite its flaws, the Global Assessment on Human Induced Soil Degradation (GLASOD) provided a database through which experts could assess susceptible dry land soil degradation in terms of spatial distribution, contributory degradation processes, and relationships to land use.

The GLASOD estimated that during the late 1980s and early 1990s approximately 1 billion hectares, equivalent to 20 percent of the susceptible dry lands, had experienced accelerated soil degradation caused by human activities. Water erosion was identified as the major physical process of degradation in 48 percent of this area and wind erosion in 39 percent. Chemical degradation (including salinization) was dominant in just 10 percent of the area, and physical changes such as compaction and crusting in just 4 percent. The severity of degradation was described by the GLASOD as strong or extreme in 4 percent of the susceptible dry lands. This figure relates to lands that have had their original biotic (relating to living organisms) functions of the soil destroyed and that are irreclaimable without major restorative measures.

The spatial character of desertification is also the subject of controversy. The spread of desertlike conditions is not, as popularly supposed, an advance over a broad front in the way that a wave overwhelms a beach. Rather, it is like a rash that tends to be localized around settlements. Fundamentally, “the extension of desert-like conditions tends to be achieved through a process of accretion from without, rather than through expansionary forces acting from within the deserts” (Mabbutt 1985, 2). This distinction is important because it influences perceptions of appropriate remedial or combative strategies.

Experts have debated whether desertification is irreversible. In many cases where ecological conditions are favorable because of the existence of such factors as deep, sandy soils, vegetation recovers after excess pressures are eliminated. The speed of recovery depends on how advanced deterioration is, the size of the area that is degraded, the nature of the soils and moisture resources, and the nature of local vegetation. Much desert vegetation is adapted to drought and to harsh conditions and often has inbuilt adaptations that enable a rapid response to improved circumstances.

Nonetheless, long-term monitoring elsewhere tends to reveal that in certain circumstances recovery is so slow and so limited that it may be appropriate to talk of “irreversible desertification.” For example, in southern Tunisia tracks made by tanks and wheeled vehicles during World War II are still apparent on the ground and in the devastated and unregenerated vegetation.

Causes of Desertification

The causes of desertification remain controversial. Experts have asked whether it is the result of temporary serious droughts, long-term climatic change, or human actions degrading the biological environments in arid zones. No doubt severe droughts do take place, and their effects become worse as human and domestic animal populations increase. The devastating drought in the Sahel (the semidesert southern fringe of the Sahara Desert) from the mid-1960s caused greater ecological stress than the droughts of 1910–1915 and 1944–1948, largely because of increasing anthropogenic pressures.

Experts have discredited the concept that climate is deteriorating through postglacial progressive desiccation. However, numerous studies of meteorological data (which in some cases date back as far as 130–150 years) do not allow experts to reach any firm conclusions on systematic long-term changes in rainfall, and the case for climatic deterioration—whether natural or aggravated by humans—is not proven. Indeed, Rapp wrote that after consideration of the evidence for the role of climatic change in desertification his conclusion was “that the reported desertization northwards and southwards from the Sahara could not be explained by a general trend towards drier climate during this century” (Rapp 1974, 29).

Woodcutting is a serious cause of vegetative decline around the area to the south of the Sahara. Many people depend on wood for domestic uses, and the collection of wood for charcoal and firewood is especially serious in the vicinity of large urban centers. Likewise, the recent drilling of wells has enabled rapid multiplication of domestic livestock numbers and large-scale destruction of the vegetation in a radius of 15–30 kilometers around them. Given this localization of degradation, amelioration schemes such as local tree planting may be partially effective, but ideas of planting massive belts as a cordon sanitaire (protective barrier) along the desert edge (whatever that is) would not halt deterioration of the land beyond this belt. The deserts are not invading from without; the land is deteriorating from within.

Clearly, therefore, a combination of human activities (e.g., deforestation, overgrazing, and plowing) with occasional series of dry years leads to the presently observed desertification. The process also seems to be fiercest not in desert interiors, but rather on the less arid marginal areas around them. The combination of circumstances particularly conducive to desert expansion can be found in semiarid and subhumid areas—where precipitation is frequent and intense enough to cause rapid erosion of unprotected soils and where humans are prone to mistake short-term economic gains under temporarily favorable climatic conditions for long-term stability.

These tendencies toward bad land-use practices partly result from the imposition of state boundaries on many traditional nomadic societies, restricting their migration routes, or from schemes implemented to encourage the nomads to become sedentary. Some of their traditional grazing lands have been taken over by cash-crop farmers. The traditional ability to migrate enabled pastoral nomads and their stock to make flexible use of available resources according to season and yearly variations in rainfall and to move away from regions that had become exhausted after a long period of use. As soon as migrations are stopped and settlements imposed, such options are closed, and severe degradation occurs.

People have suggested not only that deserts are expanding because of human activity, but also that the deserts themselves were created by human activity. People have proposed, for example, that the Thar Desert of northwest India is a postglacial and possibly a postmedieval creation, and others have suggested that the vast Sahara Desert itself is largely human made (Ehrlich and Ehrlich 1970). This proposal is not accurate. The Sahara, although it has fluctuated in extent, is many millions of years old, predates human life, and is the result of its climatic situation.

Possibly the most famous case of desertification associated with soil erosion by deflation was the Dust Bowl of the 1930s in the United States. In part the Dust Bowl was caused by a series of unusually hot, dry years that depleted the vegetation cover and made the soils dry enough to be susceptible to wind erosion, but the effects of this drought were worsened by years of overgrazing, poor farming techniques, and the rapid expansion of wheat cultivation in the Great Plains. The number of cultivated hectares doubled during World War I as tractors (for the first time) were employed in the thousands. In Kansas alone the wheat area increased from less than 2 million hectares in 1910 to almost 5 million in 1919. After the war wheat cultivation continued apace, helped by the development of the combine harvester and government assistance. Over large areas the tough sod, which had exasperated earlier homesteaders, had given way to friable (easily crumbled or pulverized) soils of high erosion potential. Drought, acting on damaged soils, created the “black blizzards.”

Dust storms are still a serious problem in parts of the United States. For example, in the San Joaquin Valley of California a dust storm caused extensive damage and erosion in 1977. More than 22 million metric tons of soil were stripped from grazing land within a twenty-four-hour period. Although the combination of drought and a high wind (as much as 300 kilometers per hour) provided the predisposing natural conditions for the stripping to occur, overgrazing and the general lack of windbreaks in the agricultural land played a more significant role. In addition, broad areas of land had recently been stripped of vegetation, leveled, or plowed up prior to planting. Other quantitatively less important factors included stripping of vegetation for urban expansion, extensive denudation of land in the vicinity of oilfields, and local denudation of land by vehicular recreation. Elsewhere in California dust yield has been considerably increased by mining of dry lake beds and by disturbance of playas (flat-floored bottoms of undrained desert basins that become at times shallow lakes).

A comparable acceleration of dust storm activity occurred in the former Soviet Union. After the “Virgin Lands” program of agricultural expansion during the 1950s, dust storm frequencies in the southern Omsk region increased on average by a factor of 2.5 and locally by factors of 5 to 6.

Desertification is not restricted to heavily populated lands with large agricultural and pastoral populations. As the examples of the U.S. Great Plains and California illustrate, high technology, non-labor-intensive land, and water use can also cause severe degradation.

Human-induced desertification is not new. Although people often focus on the Dust Bowl years of the 1930s and the current degradation of the Sahel, desertification has been the subject of great interest in the Mediterranean area since classical times. Likewise, some evidence indicates that more than four thousand years ago Mesopotamia was suffering from chemical degradation of soils and crop yield reductions as a consequence of the adoption and extension of irrigation. Nevertheless, land degradation is not the inevitable consequence of increasing population densities and land-use intensification, and many techniques are available for desert reclamation.

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