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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 may not keep up. Climatologist James Hansen predicts climate zones will shift toward the poles by 50 to 75 kilometers a year—faster than trees can naturally migrate. Species that find themselves in an unfamiliar environment will die. The 1,000-kilometer-wide strip of forest running through Canada,Russia,and Scandinavia could be cut by half. Millions of dying tress 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"ef-fect 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 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 seven 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、Impact of global warming on climate.
B、Prosperity and cheap fossil fuels.
C、Material progress and energy consumption.
D、Plants and animals in the changing climate.
答案A
解析 主旨题。文章指出“No one knows quite how they will interact with the changing climate．Millionsof dying tress would soon lead to massive forest fires，further boosting global warming．There aredozens of other possible“feedback mechanisms”，which may actually damp down global warming．Even if we were to magically stop all greenhouse-gas emissions tomorrow the impact on global climatewould continue for decades．”这说明，本文主要讲的是“全球变暖对气候的影响”。这与A符合。B、C和D只是文章部分段落的内容，不能表达文章的中心意思。
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