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  3. It’s impossible to write about how human activities are altering the climate without bringing protests that such a thing is impo

It’s impossible to write about how human activities are altering the climate without bringing protests that such a thing is impo

admin2013-09-23  27
问题     It’s impossible to write about how human activities are altering the climate without bringing protests that such a thing is impossible, that puny humanity cannot possibly alter such a gargantuan system through loading the atmosphere with additional 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(气象的)consequences. To wit; large dams seem to be altering rainfall patterns.
    Geophysicists have suspected as much for years, notes a team of scientists in a paper in the Dec. 1 issue of Eos, a publication of the American Geophysical Union. But it is becoming clearer that in addition to providing lots of water to evaporate and then return to the ground as rainfall, as scientists at MIT described in a 1996 study, dams also make local meteorological conditions more conducive to precipitation(降水).
    In particular, explained 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—specifically, the reservoir it creates—increases evaporation and therefore atmospheric moisture. That enhances the amount of con-vective(对流的)energy in the air above the reservoir. The end result; more precipitation.
    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 precipitation punctuated by drenching, flood-inducing downpours. Extreme precipitation events around large dams have increased significantly, as Hossain describes in an upcoming paper; 99th-percentile downpours in the region of a large dam have increased 4 percent per year after a large dam was built, especially in southern Africa, India, the western United States, and Central Asia. Other studies have shown how changes in land cover as seemingly innocuous as irrigating fields and draining swamps can alter local precipitation patterns, as this paper as well as this one have described.
    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 proverbial 100-year 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. A "flood-safe" dam may not be.
    As the Eos authors warn, " 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 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 acknowledgment 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 precipitation 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 50 years old by 2020.
What is the main idea of the passage?
选项 A、Human activities are changing the climate.
B、Dams make local meteorological conditions worse.
C、Dams alter local weather and make themselves unsafe.
D、Extreme precipitation events around large dams have increased.
答案C
解析 主旨题。根据全文内容可归纳本文大意为：人类修建的用来防洪的大坝会改变当地的气候，导致更多的极端降水事件；此因素再加上另一个因素：许多大坝修建的年限很长，使得大坝自身不再安全，所以[C]”大坝改变当地气候并使它们自身不再安全”是较为完整的主旨概括，故为答案。[A]作标题太宽泛，[B]、[D]概括得不全面，所以都排除。
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