Desertification

Desertification comprises a variety of natural and human processes that cause the impoverishment of ecosystems. Desertification is evident in reduced biological productivity, rapid deterioration of soil quality, and associated declines in the condition of regional human economic and social systems.

Definition of Desert Conditions

Desertification—also referred to as desertization—is a complex set of interactions between natural and cultural forces and most often occurs in the borderlands of large, natural desert environments. The broadest conventional definition of desert conditions refers to precipitation levels: Regions that receive less than 100 millimeters of precipitation per year, for example, usually exhibit commonly recognized surface features such as flat, gravelly, or pebbly tables and accumulations of sand dunes. The long-term absence of concentrated vegetation cycles and chemical weathering results in only thin, marginally fertile soils, although in places along internal drainage systems these soils may be quite productive in the short run.

The term "desert" inevitably evokes references to land and soil, but in terms of precipitation levels and their consequences, the term can also be applied to surface regimes such as glacier fields and ice caps. Annual precipitation in Antarctica, for example, is well below 100 millimeters. Both ocean surfaces, in the sense of annual precipitation input, and various ocean levels and seabed, in terms of the presence of nutrients analogous to soil on land, may be classified as desert or as undergoing forms of the desertification process, resulting from both natural and human-induced causes. Land deserts typically feature dispersed, perennial vegetation, some of which may be in a dormant state much of the time. Vegetation may be somewhat denser in regions where groundwateraquifers are near the surface, or may be absent entirely, as in major dune fields.

Human activities along the desert fringe, at least until recent times, were also remarkably accurate indicators of the boundaries of desert conditions. The 100-millimeter line of annual precipitation marks the effective limit of economical agriculture without the use of systematic irrigation. The presence of irrigation, therefore, is frequently a danger signal that human exploitation has ignored the limiting factors of arid environments.

Wind and Water Erosion

Desertification is primarily the result of wind and water erosion. The first step is usually the loss of diffuse, arid zone vegetation that normally is sufficient to protect the soil surface from dramatic erosion, or to intercept the wind transport of a sufficient amount of airborne sand and dust from other areas to balance the loss of local soil matter. This vegetation loss may occur from natural or human causes. The former may include long-term climatic shifts resulting in reductions in annual precipitation or in shorter-term climatic or meteorological phenomena such as abnormally long drought or erratic cloudbursts.

Once protective vegetation is lost, wind erosion can remove even the coarser materials in a soil layer in a matter of a few years. Much of this material will accumulate in moving dunes that become a threat to surviving marginal vegetation. Finer materials may be carried thousands of miles, even to other continents. Dust from the Sahara, for example, may be carried deep into Europe or even across the Atlantic Ocean to Florida and the Caribbean littoral. Dust layers in the Greenland ice cap have been traced to the Gobi Desert.

Erosion of soil matter exposes a surface of pebbles and gravels extremely sensitive to further damage, particularly damage resulting from human actions. In parts of the North African Sahara, vast areas that were scenes of armored engagements in World War II still exhibited ruts and surface disturbance from heavy-tracked vehicles nearly half a century later. Mass motorcycle races in the Mojave Desert of California have caused extensive, and what is likely to be long-lasting, surface damage.

Assessment of the desert environment in terms of average conditions often obscures the potential of water erosion to accelerate desertification. Hard stone or clay desert surfaces can become impermeable to water absorption by the action of raindrops on the finer soil materials, which seal the surface to the point where nearly all precipitation is lost by runoff. This condition not only destroys remnant vegetation beneath the surface by depriving it of water but also vastly increases the volume of water passing through internal drainage systems in periodic flash floods, which wreak havoc on surviving vegetation clinging to the banks of usually dry riverbeds.

Still another cause of desertification is accumulation of salts and alkaline substances in the soil through a process called salinization. Although this process may develop naturally along interior drainage systems, the most extreme cases result from prolonged irrigation. Virtually all water sources contain some level of dissolved minerals. In regions of poor drainage, an endemic characteristic of many arid and semiarid zones due to infrequent rainfall, these minerals accumulate over the years and eventually render the soil useless for agriculture, encouraging its abandonment to the forces of wind erosion. Thousands of square kilometers have been lost to productive agriculture through salinization in ancient agricultural centers such as the Near East and Pakistan, as well as in more recently exploited regions in California and Australia.

Human Exploitation of Desert Environments

In traditional or premodern societies, human exploitation of desert environments relied upon pastoral economies, wherein grazing livestock converted plant matter for human use in the form of meat, wool, and dairy products. Herders moved their animals from region to region in a form of transhumance (the seasonal movement of herds from pasture to pasture). Social mechanisms that discourage intensive environmental exploitation and continually redistribute wealth were common in these societies. The conversion of a pastoral economy from dependence on a species such as the dromedary, a solitary and desultory grazer, to social herders and voracious grass-eaters such as sheep or goats—not to mention the issue of expanding human numbers—can destroy the equilibrium in a marginal zone in a few seasons. Sheep, for example, have been known to stimulate erosion simply because entire herds repeatedly follow the same path to a water source and destroy the vegetation and soil cover along the way.

As population density soared in the twenty-first century, lower-income populations increased the practice of clearing marginal lands of vegetation for activities such as harvesting firewood. Ecologists estimate that an average-sized family living in the arid borderlands of the African-Asian desert belt consumes the wood production of 1 to 3 hectares of land each year and more than 25 million hectares per year may be denuded of trees and forests to provide fuel for growing lower-income communities.

Findings from Climatology and Meteorology

The central problem in desertification research is to determine to what degree expansion of deserts results from long-term, natural fluctuations in the global environment and to what extent the phenomenon is a result of human intervention, and to distinguish the two. All the techniques that may be applied to the reconstruction of climatic history and the tracking of meteorological patterns, therefore, pertain to desertification studies.

Because of the generally brief period covered by modern meteorological records and an imperfect knowledge of the broader patterns of global climatic change, many theories have emerged that accord varying importance to natural and human-induced factors in desertification. Some climatologists propose that during the twentieth century a global swing toward greater aridity became apparent. Northern Hemisphere records suggest an expansion of polar air masses into lower latitudes with a consequent depression of moist equatorial air masses closer to the equator, so that the convergence between these systems—which roughly marks the northernmost penetration of tropical monsoons into North Africa and the Middle East—does not come as far north as it once did.

Conversely, the input of solar energy into tropical ecosystems, which drives the monsoons, could be decreasing. The GARP Atlantic Tropical Experiment (GATE), a massive, international research effort conducted in the mid-1970s to learn more about tropical environments and weather patterns, provided a knowledge base from which alarming conclusions may be drawn concerning the rapid deforestation of the tropics because of human exploitation, the consequent decrease in photosynthesis activity and increase in the albedo of the tropics, and the overall decrease in solar energy entering tropical ecosystems that results from these changes. Appreciation of the enormous importance of the tropics, as opposed to high latitudes, in understanding middle-latitude desertification was a major outcome of GATE.

Seeking Solutions

The major objective of research into desertification is the discovery of procedures that may slow, or even reverse, what specialists in all fields agree is a relentless advance of desert frontiers. Conditions in sandy deserts with moving dunes have received particular attention in applied research because of the danger they present to settlements and installations. Careful analysis of the mineral and chemical composition as well as the physical sizes of particles, all of which may vary widely, is crucial in determining the most appropriate measures for possible fixation. Some dunes may be anchored by carefully chosen plant cover. Liquid binders and emulsions have also been tried with some success. Ironically, since intensive human exploitation is responsible for much contemporary desertification, applied research is frequently directed toward new dimensions of human adaptation to desert conditions. Results have included attempts to exploit desert plant species, inexpensive means of desalinization to utilize often plentiful sources of water in desert aquifers, and more efficient solar energy technologies; however, desert aquifers are finite and exhaustible resources.

Remote sensing techniques, using special forms of aerial photography and various optical sensors in satellite observations, now provide a season-by-season record of conditions in deserts and borderlands. Detailed satellite imagery for much of the earth extends back over forty years, providing a valuable historical perspective on environmental changes. Infrared sensors are capable of recording early germination of plant life, concentrations of various substances in the soil composition, and the presence of certain contaminants. Landforms and topographic configurations not readily apparent on the ground (a fact that may have a bearing on the presence of water or the beginnings of vegetation deterioration, or that may make certain areas particularly vulnerable to erosion), frequently reveal themselves through remote sensing. Small-scale mapping of vast areas and careful monitoring of conditions are keys to identifying desertification in its early stage when reversal may still be possible.

Established in 1994, the United Nations Convention to Combat Desertification (UNCCD) aimed to avoid, reduce, reverse, and properly document the desertification process. Such international efforts continually provide support for people negatively impacted by desertification—a group estimated to number 3.2 billion people in 2022. Other organizations have also formed to combat the issue. For example, in 2023, the DeserTech initiative partnered with the Great Green Wall to tackle issues such as rainwater collection, off-grid solar production, and roof gardening solutions across African countries.

The Sahel

Much research on desertification in the late twentieth century concentrated on the Sahel, the fragile zone on the southern margins of the Sahara where true desert conditions become transitional to the grassland environments of tropical Africa. Beginning in the 1970s, a series of calamitous droughts struck the Sahel, bringing death and misery to huge numbers of people and animals and destroying entire pastoral societies. The impact of drought in the Sahel is magnified tremendously by the fact that the greatest breadth of the African continent is precisely in this zone, so that changes in northernmost penetration of the tropical monsoons of only a few minutes of latitude can spell catastrophe over thousands of square kilometers.

Research in the Sahel demonstrates the difficulty of separating natural from human causes of desertification. In Sudan, for example, there is well-documented evidence, even in the recent colonial period, for retreat of the boundary zone between grassland and desert in some areas several hundred miles to the south and concomitant decreases in rainfall. Yet there is also a much broader picture, derived from archaeological studies, suggesting that this pattern may extend many centuries into the past and that modern changes, therefore, cannot be entirely responsible.

Despite evidence that desertification processes have been at work for many centuries, the pressure of the recent population increase on the Sahel and other marginal areas has created an atmosphere of crisis around applied research programs, so that most are directed toward fundamental changes in local practices in agriculture and animal husbandry or toward climate modification and desert reclamation. Both strategies often have the characteristic of uncontrolled experiments. Socioeconomic intervention tends to stress traditional or small-scale economic practices, often on the partly subjective conviction that they are better adapted to local conditions.

Schemes for climate and weather modification frequently require drastic environmental change. Among the more moderate proposals for the Sahel are construction of tree shelter belts, local vegetation modification, and cloud seeding designed to force the monsoon effect northward. Other schemes demand fundamental intervention in the patterns of atmospheric circulation that broadly determine the location of major desert systems. Most of these require gross changes in surface features. The creation of huge lakes in the natural drainage basins of the Sahara, through dams, evaporation control, and exploitation of groundwater aquifers, might increase precipitation levels. Many of these basins actually held such lakes during wetter conditions during the Pleistocene. One research team proposed paving large strips or "islands" of the Sahara with asphalt. The surface albedo of these areas would be much lower than sand or even vegetation, and their higher temperatures presumably would heat the air above them, promoting cloud formation and precipitation. Such a plan assumes sufficient moisture in the air at the outset, for cloud seeding cannot create moisture where none exists. Others have proposed periodic releases of carbon dust into the atmosphere over the Sahara to increase heat absorption and cloud formation. None of these schemes has been attempted, and all of them would be likely to generate scientific controversy given their impact on the environment and their unknown potential.

Vulnerability to Desertification

Desertification is not confined merely to marginal lands bordering on true deserts. Careless agriculture or animal husbandry can create near-desert conditions in vast areas of grassland, as in the case of the Dust Bowl phenomenon of the 1930s on the central plains of the United States. Enormous amounts of top-quality soil are lost each year to water and wind erosion resulting from inadequate farming practices.

Much of the land most vulnerable to desertification, however, lies in lower or tropical latitudes, where soil regimes and microenvironments are not nearly as resilient as they are in Europe or North America. In many of these areas, the human issues arising from desertification involve not so much a lowering of living standards as the very survival of local populations. They call for massive intervention schemes with multiple economic and social dimensions, usually on the assumption that the local capacity to respond to emergencies of this magnitude is limited.

Principal Terms

albedo: the fraction of visible light of electromagnetic radiation that is reflected by the properties of a given type of surface

aquifer: a water-bearing bed of rock, sand, or gravel capable of yielding substantial quantities of water to wells or springs

desalinization: the process of removing salt and minerals from seawater or from saline water occurring in aquifers beneath the land surface to render it fit for agriculture or other human use

moving dunes: collections of coarse soil materials that result from wind erosion and threaten marginal vegetation and settlements as they move across deserts

salinization: the accumulation of salts in the soil

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