The deserts, 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 desert-like conditions into areas where they did not previously exist is called desertification. It has been estimated that 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 and the subsequent accelerated erosion of the soil by wind and water. In some cases the loose soil is blown completely away, leaving a stony surface. In other cases, the finer particles 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 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. Water absorption is greatly reduced; consequently runoff is increased, resulting in accelerated erosion rates. The gradual drying of the soil caused 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 increase in desert areas is occurring largely as the result of a trend toward drier climatic conditions. Continued gradual global warming has produced an increase in aridity for some areas over the past few thousand years. The process may be accelerated in subsequent decades if global warming resulting from air pollution seriously increases.
There is little doubt, however, that desertification in most areas results primarily from human activities rather than natural processes. The semiarid lands bordering the deserts exist in a delicate ecological balance and are limited in their potential to adjust to increased 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 these stresses. During the dry periods that are common phenomena along the desert margins, 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 desertification processes: overcultivation, overgrazing, firewood gathering, and overirrigation. The cultivation of crops has expanded into progressively drier regions as population densities have grown. These regions are especially likely to have periods of severe 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 semiarid lands, where grasses are generally the dominant type of natural vegetation. The consequences 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 used for cooking and heating in many countries. The increased 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 use of dried animal waste as a substitute fuel has also hurt the soil because this valuable 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 overirrigation. Excess water from irrigation sinks down into the water table. If no drainage system exists, the water table rises, bringing dissolved salts to the surface. The water evaporates and the salts are left behind, creating a white crustal layer that prevents air and water from reaching the underlying soil.
The extreme seriousness of desertification 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 reverse the present deterioration of the surface.
原題
Paragraph 10: The extreme seriousness of desertification 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 reverse the present deterioration of the surface.
11. It can be inferred from the passage that the author most likely believes which of the following about the future of desertification?
○Governments will act quickly to control further desertification.
○The factors influencing desertification occur in cycles and will change in the future.
○Desertification will continue to increase.
○Desertification will soon occur in all areas of the world.
查看正確答案和解析
答案
11. ○3
This is an Inference question asking for an inference that can be supported by the passage. The correct answer is choice 3; the passage suggests that the author believes "Desertification will continue to increase." The last paragraph of the passage says that slowing or reversing the erosion process will be very difficult, but that it may occur in those areas that are not too affected already if rigorously enforced anti-erosion processes are implemented. Taken together, this suggests that the author is not confident this until happen; therefore, it can be inferred that he thinks erosion will continue. The passage provides no basis for inferring choices 1, 2, or 4.
沙漠已經(jīng)占據(jù)了地球陸地面積約四分之一，而且最近幾十年正以驚人的速度擴張。沙漠化是指類似沙漠的環(huán)境漫延到原本并非沙漠的區(qū)域。據(jù)估計，地球表面另外四分之一的地方正面臨沙漠化威脅。
沙漠化主要通過以下過程實現(xiàn)：首先自然植被不斷減少，隨后風(fēng)力和雨水加速了土壤的腐蝕。有的時候松散的土壤全部被風(fēng)刮走，留下石質(zhì)化的表層；其它情況下細小的沙粒可能會被吹走，而正常沙粒大小的砂子不斷堆積，從而形成移動的沙丘或者沙脊。
即便是在保留了土壤表層的區(qū)域，植被減少也已成為土壤大量吸取地下水的能力下降的典型因素。雨水對松散土壤的沖擊會把細小的粘土顆粒沖到土壤空隙中，封閉了土壤并降低土地表層水的滲透率。地表對水的吸收急劇減少，大量水資源流失，因此土壤的腐蝕率也隨即增加。地表吸收水分的能力進一步弱化使得土壤越發(fā)干燥，導(dǎo)致植被的進一步流失，于是便形成了土壤沙漠化的惡性循環(huán)。
在一些地方，沙漠面積的擴大很大程度上歸因于干燥的氣候條件。在過去的幾千年里，不斷增加的溫室效應(yīng)使得一些地方干旱問題愈發(fā)嚴(yán)重。倘若空氣污染帶來的溫室效應(yīng)繼續(xù)惡化，沙漠化進程會在未來數(shù)十年內(nèi)加速實現(xiàn)。
然而，可以肯定的是，大部分地區(qū)沙漠化主要都是由于人類活動造成，而非自然條件導(dǎo)致。沙漠邊緣的半干旱土地所處的生態(tài)平衡環(huán)境非常脆弱，環(huán)境壓力持續(xù)增加，而這些半干旱區(qū)域適應(yīng)環(huán)境壓力的能力極其有限。人口數(shù)量的增加使得人們不斷向土地施壓，依其提供食物和燃料。在濕潤的季節(jié)里，土地興許能夠應(yīng)付這些壓力。但是在干旱的季節(jié)里，在沙漠周邊的土地上，存在著這樣一個十分普遍的現(xiàn)象：人類對土地施加的壓力遠遠超過了土地自身減壓的能力，因此最終形成了沙漠。
導(dǎo)致沙漠化的主要因素有四個：過度種植，過度放牧，過分砍伐，過度灌溉。由于人口密度增加，人們對糧食作物的種植已經(jīng)擴展到日益干燥的區(qū)域進行。這些區(qū)域很有可能經(jīng)常會發(fā)生干旱，所以農(nóng)作物種植失敗是很正常的事情。大多數(shù)農(nóng)作物的種植需要事先移除天然植被，而農(nóng)作物欠收后又會留下大面積荒地，非常容易被風(fēng)力和雨水侵蝕。
在半干旱地區(qū)，草坪是主要的天然植被，家畜飼養(yǎng)是當(dāng)?shù)氐囊豁椫饕?jīng)濟活動。在一個地區(qū)過量飼養(yǎng)家畜會導(dǎo)致植被覆蓋面積減少，土地被大量踐踏和碾碎。通常，隨之而來的就是土地硬化和加速侵蝕。
在很多國家木材是用來做飯和加熱的最主要燃料。人口增加帶來的壓力促使人們大量砍伐木材，導(dǎo)致許多城市和鄉(xiāng)村周圍大面積樹木和灌木減少。同時人們大量使用烘干的動物排泄物作為替代燃料同樣對土壤不利，因為這些珍貴的土壤成分調(diào)節(jié)劑和植物營養(yǎng)資源將不會再回歸至土壤當(dāng)中。
造成土地沙漠化的最后一個主要人為因素在于人類過度灌溉導(dǎo)致土壤的鹽堿化。灌溉多余的水滲透到地下水位。假如沒有排水系統(tǒng)的存在，那么地下水位上升，把溶解的鹽分帶到土壤表面。水分蒸發(fā)后，鹽分留在了表面，形成白色的地殼層，這一地殼層阻止了空氣和水接觸地底下的土壤。
沙漠化問題異常嚴(yán)重，這是因為有佷廣闊的地區(qū)和數(shù)量龐大的人群都受到了沙漠化的影響，而且要想逆轉(zhuǎn)沙漠化的進程甚至減緩沙漠化的速度都面臨著巨大的困難。一旦土壤被侵蝕，需要再經(jīng)過幾百到上千年的時間才會產(chǎn)生新的土壤。那些大量土壤仍保存完好的地方，亟需一個嚴(yán)謹而有力的保護政策和植被覆蓋計劃來保護現(xiàn)有土地。
