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  3. Dam It’s impossible to write about how human activities are altering the climate, that trivial humanity cannot possibly alte

Dam It’s impossible to write about how human activities are altering the climate, that trivial humanity cannot possibly alte

admin2013-05-04  42
问题                                 Dam
    It’s impossible to write about how human activities are altering the climate, that trivial humanity cannot possibly alter such a giant system through loading the atmosphere with an additional 0.000115 percentage points of carbon dioxide.
    Unfortunately for the denialists, examples of how human activities can alter climate keep accumulating. The latest has nothing to do with the greenhouse effect but underlines the fact that ordinary activities can have unexpected meteorological consequences. So we infer large dams seem to be altering rainfall patterns. It is becoming clearer that in addition to providing lots of water to evaporate and then return to the ground as rainfall, dams also make local meteorological conditions more helpful to rainfall.
    In particular, explain Faisal Hossain of Tennessee Technological University, dams increase atmospheric instabilities in the vertical profile of temperature and humidity. Those instabilities arise because the presence of a dam increases evaporation and there-fore atmospheric moisture. That enhances the amount of corrective energy in the air above the reservoir. The end result: more rainfall.
    Weather records support this theoretical reasoning. For one thing, there are more thunderstorms in the vicinity of a large dam compared with before the dam was built. For another, large dams are contributing to the "when it rains, it pours" phenomenon: longer periods without rainfall punctuated by drenching, flood-inducing downpours.
    The significance of dams altering local weather is not merely another example of the power of human activities to change the climate. There is also a more practical issue. When dams are constructed, engineers make assumptions about how frequently large floods will occur, and they build the dam to withstand them. But if the uncommon flood occurs more frequently because of the very presence of a dam, that calculation is wrong, and the dam may be subjected to more frequent and more extreme flood-inducing downpours.
    As one researcher warns, it is therefore possible that a large dam may be found years later to actually have been designed for a flood with a much lower recurrence interval (or higher frequency) than originally expected because the frequency of extreme precipitation events has increased due to the reservoir’s presence. Such a possibility raises concerns about dam safety. That risk is compounded by the fact that conventional dam and reservoir design over the past century has been ’ one-way’, with no acknow-ledgment of the possible feedback mechanisms between the presence of a dam and rainfall. "Indeed, dam design protocol in civil engineering continues to assume unchanging patterns of extreme rainfall events." The risk is also compounded by the age of dams: some 85 percent of large dams in the United States will be more than 5 0 years old by 2020.
    If only the idea that "no one does anything about the weather" were true.
If a dam causes more uncommon-flood occurrences, that means______.
选项 A、the dam is certain to suffer more frequent heavy storm
B、the construction of more dame is practically needed
C、engineers’ original assumptions about flood frequency need revising
D、the dam engineers build to withstand large flood serves its purpose
答案C
解析 语义分析题。根据题干关键词uncommon—flood occurrences定位到原文第五段最后一句：But if the uncommon flood occurs more frequently because of the very presence of a dam，that calculation is wrong．．．由此可知，如果因为大坝的存在而导致更多洪水的发生，那么工程师们的估算就是错误的，故选[C]项。[A]项“大坝会遭更多暴雨袭击”，[B]项“需修建更多的大坝”和[D]项“工程师所修建的防洪坝发挥了作用”，均不正确。
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