托福 托福 02 - Desert Formation
题目
1 2 3 4 5 6 7 8 9 10 11 12 13
1.The word "threatened" in the passage is closest in meaning to
  • A.Restricted
  • B.Endangered
  • C.Prevented
  • D.Rejected
  • 正确答案:
    答案解析:
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    阅读原文 中文译文

    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 desertlike 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.

    沙漠已经占据了地球陆地面积约四分之一,而且最近几十年正以惊人的速度扩张。沙漠化是指类似沙漠的环境漫延到原本并非沙漠的区域。据估计,地球表面另外四分之一的地方正面临沙漠化威胁。 沙漠化主要通过以下过程实现:首先自然植被不断减少,随后风力和雨水加速了土壤的腐蚀。有的时候松散的土壤全部被风刮走,留下石质化的表层;其它情况下细小的沙粒可能会被吹走,而正常沙粒大小的砂子不断堆积,从而形成移动的沙丘或者沙脊。 即便是在保留了土壤表层的区域,植被减少也已成为土壤大量吸取地下水的能力下降的典型因素。雨水对松散土壤的冲击会把细小的粘土颗粒冲到土壤空隙中,封闭了土壤并降低土地表层水的渗透率。地表对水的吸收急剧减少,大量水资源流失,因此土壤的腐蚀率也随即增加。地表吸收水分的能力进一步弱化使得土壤越发干燥,导致植被的进一步流失,于是便形成了土壤沙漠化的恶性循环。 在一些地方,沙漠面积的扩大很大程度上归因于干燥的气候条件。在过去的几千年里,不断增加的温室效应使得一些地方干旱问题愈发严重。倘若空气污染带来的温室效应继续恶化,沙漠化进程会在未来数十年内加速实现。 然而,可以肯定的是,大部分地区沙漠化主要都是由于人类活动造成,而非自然条件导致。沙漠边缘的半干旱土地所处的生态平衡环境非常脆弱,环境压力持续增加,而这些半干旱区域适应环境压力的能力极其有限。人口数量的增加使得人们不断向土地施压,依其提供食物和燃料。在湿润的季节里,土地兴许能够应付这些压力。但是在干旱的季节里,在沙漠周边的土地上,存在着这样一个十分普遍的现象:人类对土地施加的压力远远超过了土地自身减压的能力,因此最终形成了沙漠。 导致沙漠化的主要因素有四个:过度种植,过度放牧,过分砍伐,过度灌溉。由于人口密度增加,人们对粮食作物的种植已经扩展到日益干燥的区域进行。这些区域很有可能经常会发生干旱,所以农作物种植失败是很正常的事情。大多数农作物的种植需要事先移除天然植被,而农作物欠收后又会留下大面积荒地,非常容易被风力和雨水侵蚀。 在半干旱地区,草坪是主要的天然植被,家畜饲养是当地的一项主要经济活动。在一个地区过量饲养家畜会导致植被覆盖面积减少,土地被大量践踏和碾碎。通常,随之而来的就是土地硬化和加速侵蚀。 在很多国家木材是用来做饭和加热的最主要燃料。人口增加带来的压力促使人们大量砍伐木材,导致许多城市和乡村周围大面积树木和灌木减少。同时人们大量使用烘干的动物排泄物作为替代燃料同样对土壤不利,因为这些珍贵的土壤成分调节剂和植物营养资源将不会再回归至土壤当中。 造成土地沙漠化的最后一个主要人为因素在于人类过度灌溉导致土壤的盐碱化。灌溉多余的水渗透到地下水位。假如没有排水系统的存在,那么地下水位上升,把溶解的盐分带到土壤表面。水分蒸发后,盐分留在了表面,形成白色的地壳层,这一地壳层阻止了空气和水接触地底下的土壤。 沙漠化问题异常严重,这是因为有佷广阔的地区和数量庞大的人群都受到了沙漠化的影响,而且要想逆转沙漠化的进程甚至减缓沙漠化的速度都面临着巨大的困难。一旦土壤被侵蚀,需要再经过几百到上千年的时间才会产生新的土壤。那些大量土壤仍保存完好的地方,亟需一个严谨而有力的保护政策和植被覆盖计划来保护现有土地。

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    最新提问
    • wx_6697
      觉得B C 意思一样,不知道选哪个
    • wx_5576
      这道题C为什么对,E为什么不对?
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      B为什么不能选啊?
    • wx_6697
      TPO30 passage 2 Q5我选的 D,不明白为啥不对?
    • wx_6697
      鑫哥,TPO6passage3Q5 答案是给错了吗?好多人都选A
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      这题也很容易选错选成了D
    • wx_6697
      这道题A为什么错了
    • 芊儿
      为什么这道题不选c??a中的variety不是应该对应文中的differentiating 吗??求解!
    • wx_1000
      这道题不选E是因为太细节了吗
    • 王金阁
      这个题为什么不选C啊。。。
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      这道题的D选项不是和文中的better able to reproduce in open settings相对应么??
    • 风荨火
      有大佬解释一下这个为啥选D嘛?
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    • wx_6697
      鑫哥,这道题D是从哪里看出来的
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      F选项的weather-related destruction在哪里体现了呢?原文最后一段的开头Among the costs里的costs是不是打错了?应该是coast?
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      这题为什么选C?
    • 小雨淅沥哗啦的下
      小雨淅沥哗啦的下 去解答 去解答
      B哪里错了
    • 小雨淅沥哗啦的下
      小雨淅沥哗啦的下 去解答 去解答
      B为啥不对
    • 李浩然
      B选项错误,是因为残缺么?
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      鑫哥,从哪里看出来这个masks 不是use呀,原文说了wear呀
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    • wx_2065
      鑫哥,想知道E错在哪里?
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      鑫哥,B选项 cannot extended to earlier geological periods. 原文说的意思是后来的进化无法估计吧
    • wx_2163
      B为什么不选
    • wx_7780
      鑫哥,这个哪里看不use了。BD是修饰错,C是无中生有,怎么能直接选出A?
    • 100
      看到第一句话,以为是中心句就选了A... 为什么不能选A呢
    • 100
      为什么选b?
    • gu33
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    • 我是啦啦啦
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      这个题A哪里错了?是因为主语不对吗?这个C比A多一步推理啊
    • haiyuqiao
      鑫哥,D选项里的19世纪出现了很多假设,原文中并没有提到啊
    • wx_7060
      为什么选a 呢。我觉得a是细节。F哪错了?
    • wx_1105
      我想问一下,这道题为什么不能选A呢?
    • wx_8122
      D为什么不选
    • wx_1655
      f选项哪里说了
    • chaulaw
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    • chaulaw
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    • wx_6697
      鑫哥,这道题答案是不是错了,好多人选D 我也选的D求解答
    • wx_6697
      这道题应该是一道易错题,每个选项的都有,然而我选的A错了,求解
    • wx_6697
      鑫哥,这题的C是怎么得到的?B也没有找到啊?难道不是应该选B
    • wx_6697
      鑫哥,还有这个,好多人选A答案是不是错了
    • wx_6697
      求问D是从哪里得出来的,我选的B呀哎呀
    • wx_4185
      it is difficult to say how far they were intended to be portraits rather than generalized images 这句话怎么理解呢
    • 此楠楠
      请问下这个插入题怎么选的呢?
    • 此楠楠
      求鑫哥讲解下A选项。。。 Even though in error, Joly’s calculations clearly supported those geologists who insisted on an age for Earth far in excess of a few million years.