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  3. Carbon dioxide is a "greenhouse" gas, which means that it helps to trap heat in the atmosphere, (46)More carbon dioxide on the f

Carbon dioxide is a "greenhouse" gas, which means that it helps to trap heat in the atmosphere, (46)More carbon dioxide on the f

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问题     Carbon dioxide is a "greenhouse" gas, which means that it helps to trap heat in the atmosphere, (46)More carbon dioxide on the face of it means a hotter earth and that might lead to heaving seas, scorching summers, dying forests, and a watery end to the world’s coastal cities. But carbon dioxide is also an inevitable by-product of burning the fuels—coal, off and natural gas—that make an industrial way of life possible. The results of cutting its production could therefore be profound. People in rich countries might have to change their comfortable existence in order to consume less energy. (47)Those in countries trying to become rich might see their own aspirations to such comforts confounded, or at least delayed. It is therefore important to ask exactly how real the threat of global warming is, just what sort of climate change it implies, how imminently that change can be expected, and what the cheapest way to deal with any adverse consequences it brings would actually be.
    (48)That the greenhouse effect exists is not a matter of dispute. Joseph Fourier, a French physicist, theorized as far back as 1827 that the earth’s atmosphere acts rather like the glass of a plant-breeder’s hothouse: in other words, the air lets in the sun’s heat while slowing its release back into space. (49)Without this effect, the earth would be some 30’C colder than it is, and life would scarcely exist.
    The amount of carbon dioxide in the atmosphere has been rising for more than a century, as the use of fossil fuels has become widespread. And human activity also puts other greenhouse gases into the atmosphere. Though released in smaller quantities, some of these are more potent in their warmth-inducing effects than carbon dioxide. All told, manmade emissions account for slightly less than 4% of all greenhouse gases.
    That may not sound a lot, but this 4% is reckoned to have enhanced the earth’s average temperature by between 0.3℃ and 0.6℃ over the past years. And in matters climatic, small changes can sometimes have large consequences. (50)The glaciers that rumbled over Europe and North America during the last ice age, for example, were triggered by a fall of 2℃ in the average summer temperature around 115,000 years ago.

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答案仅举一例，在上一个冰河时代就横扫整个欧洲和北美洲的冰川就是由大约十一万五千年前夏季平均气温下降了2℃而引发的。
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