托福TPO2阅读真题及答案Part1
托福TPO作为托福的模考工具，它的题目对于我们备考托福很有参考价值，为了帮助大家备考，下面小编给大家整理了托福TPO2阅读真题及答案Part1，望喜欢！
托福TPO2阅读真题文本Part1
Dert Formation
The derts, which already occupy approximately a fourth of the Earth's land surface, have in recent decades been increasing at an alarming pace. The expansion of dert like conditions into areas where they did not previously exist is called dertification. It has been estimated that an additional one-fourth of the Earth's land surface is threatened by this process.
Dertification is accomplished primarily through the loss of stabilizing natural vegetation and the subquent accelerated erosion of the soil by wind and water. In some cas the loo soil is blown completely away, leaving a stony surface. In other cas, the finer particl
es may be removed, 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 the loss of the soil's ability to absorb substantial quantities of water. The impact of raindrops on the loo soil tends to transfer fine clay particles into the tiniest soil spaces, aling them and producing a surface that bamboos
allows very little water penetration. Water absorption is greatly reduced; conquently runoff is incread, resulting in accelerated erosion rates. The gradual drying of the soil caud by its diminished ability to absorb water results in the further loss of vegetation, so that a cycle of progressive surface deterioration is established.
In some regions, the increa in dert areas is occurring largely as the result of a trend toward drier climatic conditions. Continued gradual global warming has produced an increa in aridity for some areas over the past few thousand years. The process may be accelerated in subquent decades if global warming resulting from air pollution riously increas.
There is little doubt, however, that dertification in most areas results primarily from human activities rather than natural process. The miarid lands bordering the derts exist in a delicate ecological balance and are limited in their potential to adjust to incread environmental pressures. Expanding populations are subjecting the land to increasing pressures to provide them with food and fuel. In wet periods, the land may be able to respond to the stress. During the dry periods that are common phenomena along the dert margins, though, the pressure on the land is often far in excess of its diminished capacity, and dertification results.
Four specific activities have been identified as major contributors to the dertification process: overcultivation, overgrazing, firewood gathering, and overirrigation. The cultivation of crops has expanded into progressively drier regions as population densities have grown. The regions are especially likely to have periods of vere dryness, so that crop failures are common. Since the raising of most crops necessitates the prior removal of the natural vegetation, crop failures leave extensive tracts of land devoid of a plant cover and susceptible to wind and water erosion.
The raising of livestock is a major economic activity in miarid lands, where grass are generally the dominant type of natural vegetation. The conquences of an excessive number of livestock grazing in an area are the reduction of the vegetation cover and the trampling and pulverization of the soil. This is usually followed by the drying of the soil and accelerated erosion.
Firewood is the chief fuel ud for cooking and heating in many countries. The incread pressures of expanding populations have led to the removal of woody plants so that many cities and towns are surrounded by large areas completely lacking in trees and shrubs. The increasing u of dried animal waste as a substitute fuel has also hurt the soil becau this valuable soil conditioner and source of plant nutrients is no longer being returned to the land.
The final major human cau of dertification is soil salinization resulting from overirrigation. Excess water from irrigation sinks down into the water table. If no drainage system exists, the water table ris, bringing dissolved salts to the surface. The water ev
aporates and the salts are left behind, creating a white crustal layer that prevents air and water from reaching the underlying soil.
The extreme riousness of dertification results from the vast areas of land and the tremendous numbers of people affected, as well as from the great difficulty of reversing or even slowing the process. Once the soil has been removed by erosion, only the passage of centuries 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 it possible to rever the prent deterioration of the surface.
