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.

　　17. The primary purpose of the passage is to

　　 interpret data

　　 explain research methodology

　　 evaluate a conclusion

　　 suggest a new technique

　　 attack a theory

　　18. According to the passage, which of the following statements about methane is true?

　　 Methane is found in marine sediments.

　　 Methane is more effective than carbon dioxide in absorbing radiant heat.

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