Desertification

I

Introduction

Desertification, term applied to land degradation in drylands resulting mainly from adverse human impact. “Land” in these terms includes soil and local water resources, the land surface and vegetation or crops, while degradation implies a reduction of resource potential.

The term was first coined in 1949 by a French forester working in West Africa who used it to describe the gradual clearance of forests in humid areas on the edge of the Sahara Desert until the trees disappeared and the area became more desert-like. Desertification has subsequently been recognized as one of a series of processes that affect drylands all over the world. These processes include water erosion and wind erosion, as well as sedimentation by those agents, long-term reduction in the amount or diversity of natural vegetation, and salinization and sodication.

Desertification was arguably the first environmental issue to be recognized as taking place on a global scale, a recognition that was formalized at the United Nations (UN) Conference on Desertification, held in Nairobi in 1977. Since then, the UN Environment Programme (UNEP) has been charged with coordinating a global attempt to combat the problem. According to UNEP estimates made in 1992, some 3,590 million hectares (8,870 million acres) or 35.9 million sq km (13.9 million sq mi) are affected world-wide, most of this being in the form of degraded vegetation on land used for grazing.

II

Modes of Desertification

So-called “overgrazing” is a result of too much livestock being kept on a given area of pasture resulting in the loss of edible species and the consequent encouragement of inedible species. If excessive grazing pressure continues, the loss of vegetation cover can result in soil erosion. Other commonly quoted ways in which human mismanagement causes desertification include “overcultivation”, in which soil is exhausted by nutrient loss and erosion, the excessive clearance of vegetation, often for fuelwood, and poor management of irrigation schemes which results in salinization of soils.

Overcultivation occurs due to the shortening of periods when the land is left free from cultivation (“fallow”), or from the use of mechanical techniques which cause widespread loss of soil. A classic case of overcultivation leading to large-scale wind erosion of soils occurred in the infamous Dust Bowl of the Great Plains of the United States in the 1930s. Here, semi-arid grasslands were ploughed up for cereal cultivation using deep ploughing techniques developed in the more temperate latitudes of western Europe. When drought hit the Great Plains in 1931, wind erosion produced dust storms on an unprecedented scale. A similar ecological catastrophe occurred in the same way after the Virgin Lands scheme of the 1950s in the former Soviet Union.

Forest and woodland is cleared for a variety of motives, to create agricultural and pasture land for example, but the most serious cause of desertification in this respect is the so-called “fuelwood crisis” which is characteristic of many drylands in the developing world. The collection of fuelwood from urban hinterlands in the Sahel, the most severely affected region, has resulted in the almost total loss of trees around major cities. Examples include Ouagadougou (Burkina Faso) and Dakar (Senegal), while the radius of the treeless zone around Khartoum in Sudan is 90 km (56 mi). Salinization is one of the clearest examples of human-induced desertification, affecting about one-fifth of all irrigated cropland in Australia and the United States, and one-third in countries such as Egypt, Pakistan, and Syria. The proportion is one-half in Iraq. Excessive concentrations of salts in irrigated soils adversely affect crop yields and can ultimately kill plants.

III

Causes of and Remedies for Desertification

Distinguishing between the effects of poor resource management in drylands and the naturally high variability of resource availability characteristic of such areas is by no means easy. Deserts and their margins are typically dynamic on timescales relevant to human lifetimes, a dynamism driven by precipitation that is highly variable from day to day, season to season, and during drought periods which may last for decades. Hence it is often difficult in practice to say whether true degradation of resources has occurred in the Sahel, for example, during the more-or-less permanent drought that has gripped the area since the late 1960s, because loss of vegetation cover and soils is at least partly due to natural climatic factors. These types of problems, combined with the inaccuracy of many estimates of the absolute areas affected by desertification, have led some experts to suggest that desertification as a major global environmental problem has been greatly exaggerated.

Controversy also surrounds some of the attempts that have been made to combat desertification. A common misconception of the phenomenon as an advancing front of sand dunes has spawned “green belt” projects in which lines of trees are planted to halt the desert's advance. Proposals have been made for such green belts to be planted around the entire Sahara Desert, for example. While mobile dunes do cause problems in some specific areas, as in some parts of Saudi Arabia, they are not the widespread problem so often perceived.

Changes in the academic and institutional approaches to solving desertification problems have occurred in recent years. One area in which previously conventional thinking has been reassessed is overgrazing. Ideas of “carrying capacity” developed in less variable environments may not be applicable to the highly dynamic dryland scene since natural changes mean that the amount of grazing available is in a constant state of flux. Further, social mechanisms developed by pastoral peoples who have kept dryland herds for many generations usually prevent overgrazing before degradation occurs.

Generally, there has been too great a reliance on technical solutions to desertification problems, a realization that has generated some new approaches to solving the problems of dryland degradation in recent years. Population growth, social, economic, and political factors often lie at the heart of the growing difficulties faced by communities in dryland areas. New emphasis is now being placed upon local community participation, redeployment of traditional coping strategies in times of environmental stress, such as drought, and tackling problems which stem from the marginalization of rural inhabitants by traditionally urban-based governments.

The importance of non-physical factors in encouraging unsustainable use of dryland resources is illustrated by the case of the Great Plains, mentioned earlier, where wind erosion on a comparable scale to that of the 1930s occurred again in the 1970s. Large tracts of marginal land were put to wheat cultivation in the early 1970s driven by high exports to the then Soviet Union and encouraged by a federal government incentive which paid farmers according to the area planted, irrespective of whether a crop was harvested or not. When drought hit the area in 1975, large-scale dust storms were seen again. The worst single event occurred in the Portales Valley of New Mexico in February 1977. Soil dust tracked by satellite imagery obscured 400,000 sq km (154,000 sq mi) of the ground surface of the south-central United States.