Land Degradation and Desertification

Arid and semi-arid drylands compose one third of the land surface of the world and are home to about 20% of the human population.1 The vast majority of these drylands consist of rangelands (~88%), whereas the rest are classified as rainfed (3%) and irrigated croplands (9%) (Table 6.1). The rapid growth of populations in many of these regions, often in conjunction with imprudent land management, has led to increased social vulnerability and to rapid land degradation. Soils in drylands are especially vulnerable to wind and water erosion, loss of organic matter, decline in fertility, salinization, and compaction. This is particularly evident when the natural vegetative cover is reduced through human activities such as intensive livestock grazing, excessive cultivation, urbanization, and other land uses. While global rates of land degradation and the total areas affected are difficult to estimate,2,3 there is ample evidence that extensive areas of the world's drylands have experienced some form of chronic degradation during the last century. It is estimated4,5 that approximately 80% of the world's rangelands, 60% of rainfed croplands, and 30% of irrigated croplands are threatened by various types of degradation, generally referred to as "desertification." Thus, desertified drylands make up about 65-70% of the total dryland area of the globe (arid, semiarid, and dry subhumid regions, but excluding hyperarid regions).6 All three of the developing regions of the world—Africa, Asia, and Latin America—have similar percentages of land degradation (Table 6.1).

The two major desertification drivers— climate change and human activities—have ecological and social impacts at various temporal and spatial scales, ranging from local (and short term) to global (and long term) (Fig. 6.1). Carbon, water, and trace gas budgets may be significantly altered; losses of vegetation may modify regional albedo, raise air temperatures, and increase wind-borne dust; all of these changes have the potential to act in concert to affect global biogeochemistry, radiation balance, and climate. The societal consequences of land degradation are equally serious. The fate of rural people in drylands is dependent on the effective use of natural resources, e.g., water, soils, plants, livestock and wildlife. In spite of this, over large areas natural vegetation continues to be degraded, soils are eroding, and the capacity of the land to support livestock and wild herbivores is being reduced. Combined with complex political, social, and economic factors, which often tend to have equally important roles, adverse human impacts are inevitable.1

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