Environmental Problems, Their Causes, and Sustainability 1

ModerateSevereVery severeFigure 14-9 Natural capital degradation: desertification of arid and semiarid lands. It is caused by a combinationof prolonged drought and human activities that expose soil to erosion. (Data from UN EnvironmentProgramme and Harold E. Drengue)topsoil. The process can be moderate (a 10–25% drop inproductivity), severe (a 25–50% drop), or very severe (adrop of 50% or more, usually creating huge gullies andsand dunes). Note that only in extreme cases does desertificationlead to what we call desert.Over thousands of years the earth’s deserts haveexpanded and contracted, mostly because of naturalclimate changes. However, human activities can acceleratedesertification in some parts of the world (Figure14-9). Study Figure 14-9 to find out the areas of theworld most affected by desertification. Is it a problemwhere you live?According to a 2003 UN conference on desertification,about a third of the world’s land and 70% of alldrylands is suffering from the effects of desertification.UN officials estimate that this loss of soil productivitythreatens the livelihoods of at least 250 million peoplein 110 countries (70 in Africa). China is facing seriousdesertification, as its portion of the Gobi Desert expandedby an area half the size of Pennsylvania between1994 and 1999.Figure 14-10 summarizes the major causes andconsequences of desertification. We cannot controlwhen or where prolonged droughts may occur, but wecan reduce overgrazing, deforestation, and destructiveforms of planting, irrigation, and mining that leavesoil barren. We can also restore land suffering from desertificationby planting trees and grasses that anchorsoil and hold water.How Do Excess Salts and Water DegradeSoils? Crop Losses from Too Much Saltand WaterRepeated irrigation can cause loss of cropproductivity by salt buildup in the soil andwaterlogging of crop plants.The one-fifth of the world’s cropland that is irrigatedproduces almost 40% of the world’s food. But irrigationhas a downside. Most irrigation water is a dilutesolution of various salts, picked up as the water flowsover or through soil and rocks. Irrigation water not ab-282 CHAPTER 14 Food and Soil Resources