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the deserts, which already occupy approximately a fourth of the earth's land surface, have inrecent decades been increasing at an alarming pace. the expansion of desertlike conditionsinto areas where they did not previously exist is called desertification. it has been estimatedthat an additional one-fourth of the earth's land surface is threatened by this process.

desertification is accomplished primarily through the loss of stabilizing natural vegetation andthe subsequent accelerated erosion of the soil by wind and water. in some cases the loose soilis blown completely away, leaving a stony surface. in other cases, the finer particles may beremoved, while the sand-sized particles are accumulated to form mobile hills or ridges of sand.

even in the areas that retain a soil cover, the reduction of vegetation typically results in theloss of the soil's ability to absorb substantial quantities of water. the impact of raindrops onthe loose soil tends to transfer fine clay particles into the tiniest soil spaces, sealing them andproducing a surface that allows very little water penetration. water absorption is greatlyreduced; consequently runoff is increased, resulting in accelerated erosion rates. the gradualdrying of the soil caused by its diminished ability to absorb water results in the further loss ofvegetation, so that a cycle of progressive surface deterioration is established.

in some regions, the increase in desert areas is occurring largely as the result of a trendtoward drier climatic conditions. continued gradual global warming has produced an increase inaridity for some areas over the past few thousand years. the process may be accelerated insubsequent decades if global warming resulting from air pollution seriously increases.

there is little doubt, however, that desertification in most areas results primarily from humanactivities rather than natural processes. the semiarid lands bordering the deserts exist in adelicate ecological balance and are limited in their potential to adjust to increasedenvironmental pressures. expanding populations are subjecting the land to increasingpressures to provide them with food and fuel. in wet periods, the land may be able to respondto these stresses. during the dry periods that are common phenomena along the desertmargins, though, the pressure on the land is often far in excess of its diminished capacity,and desertification results.

four specific activities have been identified as major contributors to the desertificationprocesses: overcultivation, overgrazing, firewood gathering, and overirrigation. the cultivationof crops has expanded into progressively drier regions as population densities have regions are especially likely to have periods of severe dryness, so that crop failures arecommon. since the raising of most crops necessitates the prior removal of the naturalvegetation, crop failures leave extensive tracts of land devoid of a plant cover and susceptibleto wind and water erosion.

the raising of livestock is a major economic activity in semiarid lands, where grasses aregenerally the dominant type of natural vegetation. the consequences of an excessive numberof livestock grazing in an area are the reduction of the vegetation cover and the tramplingand pulverization of the soil. this is usually followed by the drying of the soil and acceleratederosion.

firewood is the chief fuel used for cooking and heating in many countries. the increasedpressures of expanding populations have led to the removal of woody plants so that manycities and towns are surrounded by large areas completely lacking in trees and shrubs. theincreasing use of dried animal waste as a substitute fuel has also hurt the soil because thisvaluable soil conditioner and source of plant nutrients is no longer being returned to the land.

the final major human cause of desertification is soil salinization resulting from water from irrigation sinks down into the water table. if no drainage system exists, thewater table rises, bringing dissolved salts to the surface. the water evaporates and the saltsare left behind, creating a white crustal layer that prevents air and water from reaching theunderlying soil.

the extreme seriousness of desertification results from the vast areas of land and thetremendous numbers of people affected, as well as from the great difficulty of reversing oreven slowing the process. once the soil has been removed by erosion, only the passage ofcenturies or millennia will enable new soil to form. in areas where considerable soil still remains,though, a rigorously enforced program of land protection and cover-crop planting may make itpossible to reverse the present deterioration of the surface.

paragraph 1: the deserts, which already occupy approximately a fourth of the earth's landsurface, have in recent decades been increasing at an alarming pace. the expansion ofdesertlike conditions into areas where they did not previously exist is called desertification. ithas been estimated that an additional one-fourth of the earth's land surface is threatened bythis process.

○restricted

○endangered

○prevented

○rejected

paragraph 3: even in the areas that retain a soil cover, the reduction of vegetation typicallyresults in the loss of the soil's ability to absorb substantial quantities of water. the impact ofraindrops on the loose soil tends to transfer fine clay particles into the tiniest soil spaces,sealing them and producing a surface that allows very little water penetration. waterabsorption is greatly reduced; consequently runoff is increased, resulting in acceleratederosion rates. the gradual drying of the soil caused by its diminished ability to absorb waterresults in the further loss of vegetation, so that a cycle of progressive surface deteriorationis established.

○increased stony content

○reduced water absorption

○increased numbers of spaces in the soil

○reduced water runoff

paragraph 5: there is little doubt, however, that