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  3. Aided by the recent ability to analyze samples of air trapped in glaciers, scientists now have a clearer idea of the relationshi

Aided by the recent ability to analyze samples of air trapped in glaciers, scientists now have a clearer idea of the relationshi

admin2015-01-31  6
问题     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.
The primary purpose of the passage is to______.
选项 A、interpret data
B、explain research methodology
C、evaluate a conclusion
D、suggest a new technique
答案A
解析 主旨题。根据文章第一、二、四段的首句Aided by the recent ability toanalyze samples of air trapped in glaciers，scientists now have a clearer idea…，Datafrom the Vostolk core also indicate that…．Other data from the Vostok core show that…可以把握文章的主要目的是科学家通过对困在冰川中的空气样品进行分析．对大气构成与全球气温变化之间的关系进行解释说明。简而言之，就是“解释数据”，和A选项表达一致。
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