2. 外语
  3. Scientific consensus is that humans first began to have a warming effect on Earth’s climate within the past century, after coal-

Scientific consensus is that humans first began to have a warming effect on Earth’s climate within the past century, after coal-

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问题     Scientific consensus is that humans first began to have a warming effect on Earth’s climate within the past century, after coal-burning factories, power plants, and motor vehicles began releasing carbon dioxide (CO2) and other greenhouse gases in significant quantities into the air. However, evidence suggests that human agricultural activities may have had such an effect much earlier: concentrations of CO2 started rising about 8000 years ago, even though natural trends indicate they should have been dropping; methane levels rose similarly about 3,000 years later. Without these rises, however, current temperatures in northern parts of North America and Europe would be cooler by three to four degrees Celsius—enough to inhibit agriculture—and an ice age would probably have begun several thousand years ago in northeastern Canada.
The author mentions “natural trends” most likely in order to
选项 A、propose a possible explanation for why current temperatures in North America and Europe are not as low as they might otherwise be
B、explain why levels of methane in Earth’s atmosphere began to rise approximately 5000 years ago
C、suggest that Earth’s climate would become even warmer without the advent of human agriculture activities
D、identify factors that may have contributed to the rise of greenhouse gases in Earth’s atmosphere approximately 8000 years ago and again some 3000 years later
E、support the claim that human activity may have played a role in the rise in CO2 and methane levels in the atmosphere thousands of years ago.
答案E
解析 本题属于功能题，正确答案是定位句3s版本，即人类的农业活动在更早之前已经导致全球变暖了，因此选项E正确。另外，作者既然提到了自然趋势，那么就是用来跟人类活动相对比的。自然趋势下二氧化碳浓度应该是下降的，但实际上二氧化碳浓度上升了，这种反差也是为了突出人类活动对气体浓度的影响。A项无法解释为什么北美和欧洲温度高，因为自然趋势下温室气体浓度应该是下降的。B项无法解释为什么甲烷浓度升高，因为自然趋势下温室气体浓度应该是下降的。C项表述相反，在没有人类农业活动时，大气应该跟随自然趋势一起降温。D项无法解释浓度上升，因为自然趋势下气体浓度应该是下降的。
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