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  3. There are hidden factors which scientists call "feedback mechanisms". No one knows quite how they will interact with the changin

There are hidden factors which scientists call "feedback mechanisms". No one knows quite how they will interact with the changin

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问题     There are hidden factors which scientists call "feedback mechanisms". No one knows quite how they will interact with the changing climate. Here’s one example: plants and animals adapt to climate change over centuries. At the current estimate of half a degree centigrade of warming per decade, vegetation(植物)may not keep up. Climatologist James Hansen predicts climate zones will shift toward the poles by 50 to 75 kilometres a year— faster than trees can naturally migrate. Species that find themselves in an unfamiliar environment will die. The 1,000-kilometre-wide strip of forest running through Canada, the USSR and Scandinavia could be cut by half. Millions of dying trees would soon lead to massive forest fires, releasing tons of CO2 and further boosting global warming.
    There are dozens of other possible "feedback mechanisms". Higher temperatures will fuel condensation and increase cloudiness, which may actually damp down global warming. Others, like the "albedo" effect, will do the opposite. The "albedo" effect is the amount of solar energy reflected by the earth’s surface. As northern ice and snow melts and the darker sea and land pokes(戳)through, more heat will be absorbed, adding to the global temperature increase.
    Even if we were to magically stop all greenhouse-gas emissions tomorrow the impact on global climate would continue for decades. Delay will simply make the problem worse. The fact is that some of us are doing quite well the way things are. In the developed world prosperity has been built on 150 years of cheap fossil fuels.
    Material progress has been linked to energy consumption. Today 75 percent of all the world’s energy is consumed by a quarter of the world’s population. The average rich world resident adds about 3. 2 tons of CO2 yearly to the atmosphere, more than four times the level added by each Third World citizen. The US, with just 7 percent of the global population, is responsible for 22 percent of global warming.
Which of the following is the main topic of the passage?
选项 A、Material Progress and Energy Consumption.
B、Prosperity and Cheap Fossil Fuels.
C、Impact of Global Warming on Climate.
D、Plants and Animals in the Changing Climate.
答案C
解析 综观全文，第一段作者主要讲到：“反馈机制”以及这种“反馈机制”如何与变化的气候进行相互影响和相互作用，在现在没有人能够完全了解。第二段讲到：不同的反馈机制，有的抑制全球变暖，有的则加速全球变暖。第三段讲到：温室气体的排放对全球变暖问题是一个重要的方面。第四段讲到：能源的消耗问题对全球变暖问题的影响也是一个重要方面。可见文章主要讲全球变暖对气候的影响。C项符合文章的意思。A、B、D项分别是第四段、第三段、第一段中所提到的细节和相关的例子，都不能概括全文。所以本题正确答案为C。
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