单选题 Aided by the recent ability to analyze samples of air trapped in glaciers, scientists now have a clearer idea of the relationship between atmospheric composition and global temperature changing over the past 160,000 years. In particular, determination of atmospheric composition during periods of glacial expansion and retreat (cooling and warming) is possible using data from the 2,000 meter Vostok ice core drilled in Antarctica. The technique involved is similar to that used in analyzing cores of marine sediments, where the ratio of the two common isotopes of oxygen, 180 and 160, accurately reflects past temperature changes. Isotopic analysis of oxygen in the Vostok core suggests mean global temperature fluctuations of up to 10 degrees centigrade over the past 160,000 years.
Data from the Vostok core also indicate that the amount of carbon dioxide has fluctuated with temperature over the same period: the higher the temperature, the higher the concentration of carbon dioxide and the lower the temperature, the lower the concentration. Although change in carbon dioxide content closely follows change in temperature during periods of deglaciation, it apparently lags behind temperature during periods of cooling. The correlation of carbon dioxide with temperature, of course, does not establish whether changes in atmospheric composition causing the warming and cooling trends or were caused by them.
The correlation between carbon dioxide and temperature throughout the Vostok record is consistent and predictable. The absolute temperature changes, however, are from 5 to 14 times greater than would be expected on the basis of carbon dioxide's own ability to absorb infrared radiation, or radiant heat. This reaction suggests that, quite aside from changes in heat-trapping gases, commonly known as greenhouse gases, certain positive feedbacks are also amplifying the temperature change. Such feedbacks might involve ice on land and sea, clouds, or water vapor, which also absorb radiant heat.
Other data from the Vostok core show that methane gas also correlates closely with temperature and carbon dioxide. The methane concentration nearly doubled, for example, between the peak of the penultimate glacial period and the following interglacial period. Within the present interglacial period it has more than doubled in just the past 300 years and is rising rapidly. Although the concentration of atmospheric methane is more than two orders of magnitude lower than that of carbon dioxide, it cannot be ignored: the radiative properties of methane make it 20 times more effective, molecule for molecule, than carbon dioxide in absorbing radiant heat. On the basis of a simulation model that climatological researchers have developed, methane appears to have been about 25 percent as important as carbon dioxide in the warming that took place during the most recent glacial retreat 8,000 to 10,000 years ago.

单选题 The primary purpose of the passage is to ______.

单选题 According to the passage, which of the following statements best describes the relationship between carbon dioxide and global temperature?

【正确答案】 C

【答案解析】细节题。根据the relationship between carbon dioxide and global temperature定位到文章第二段。Although change in carbon dioxide content closely follows change in temperature during periods of deglaciation, it apparently lags behind temperature during periods of cooling.“尽管在冰川消退期，二氧化碳含量的变化紧随着气温的变化，但在降温期，二氧化碳含量的变化显然滞后于气温。”说明降温期二氧化碳含量先升后降，比气温下降慢一步，和选项C表述一致，所以C为正确答案。同时也排除了A“二氧化碳对气温变化做出的反应是迅速的”和B“二氧化碳含量仅在降温期和气温有关”。D“二氧化碳比气温升得更快”属于无中生有。所以答案选C。

单选题 The author mentions 'certain positive feedbacks' (Line 6, Para.3) in order to indicate that

【正确答案】 C

【答案解析】细节题。根据第三段This reaction suggests that, quite aside from changes in heat-trapping gases, commonly known as greenhouse gases, certain positive feedbacks are also amplifying the temperature change. Such feedbacks might involve ice on land and sea, clouds, or water vapor, which also absorb radiant heat. “这种关系表明，除了捕热气体(即通常所称为的温室气体)以外，某些正反馈也会扩大温度变化。此类反馈可能涉及陆地上和海洋中的冰、云或水蒸气，它们也都会吸收辐射热。”可知不仅仅是温室气体导致了温度升高，某些正反馈也是原因之一，C选项和原文表述一致，所以答案选C。

单选题 According to the passage, which of the following statements about methane is true?

【正确答案】 B

【答案解析】细节题。根据methane定位到第四段。根据Although the concentration of atmospheric methane is more than two orders of magnitude lower than that of carbon dioxide, it cannot be ignored: the radiative properties of methane make it 20 times more effective, molecule for molecule, than carbon dioxide in absorbing radiant heat.“尽管大气甲烷的含量要比二氧化碳的含量低两个多数量级，但它仍不能被忽视：就分子与分子相对比，甲烷的辐射特性使其在吸收辐射热这方面，要比二氧化碳强出20倍。”说明甲烷在吸收辐射热这方面比二氧化碳有效，和选项B的表述一致，所以答案是B。A“甲烷是在海洋沉积物中发现的”原文没有提及，C是对原文“甲烷的含量几乎翻了一倍。在目前的间冰期范围内，它在过去短短的300年就已经增加了两倍多，并正在迅速增长”的误解，D“二氧化碳含量越高，甲烷含量越低”也没有提及。