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Mysterious night-shining clouds that light up the polar skies have become more shining and frequent in recent years and climate

admin2015-04-10  68
问题     Mysterious night-shining clouds that light up the polar skies have become more shining and frequent in recent years and climate change may be the culprit, scientists announced. So-called noctilucent clouds, which streak across the sky in sparking colors during polar summers, are ten times brighter than previously believed, according to recent data from NASA’s AIM satellite. The data, collected during two polar cloud seasons, also suggest the formations appear daily, are more widespread, and have started to form at lower latitudes than before.
    Night-shining clouds are made mostly of ice particles that form when water vapor condenses onto atmospheric dust. The AIM mission is the "first detailed exploration of the clouds, which form about 50 miles above Earth’s surface—"literally on the edge of space," AIM principal investigator said. The air there is a thousand times drier than the Sahara, and the temperature can drop to minus 235 degrees Fahrenheit—ideal conditions for creating clouds. A likely explanation for the clouds’ surge is that temperatures in the upper atmosphere have gotten even colder due to the build-up of carbon dioxide in the atmosphere. Yearly emissions of carbon dioxide levels have grown about 20 percent since 1990. Most scientists agree the build-up is causing climate change and heating up Earth’s surface. In the upper atmosphere, however, carbon dioxide does not act like a thick, heat-trapping blanket.
    Instead it forms a thin layer, allowing heat to escape, temperatures to drop, and ice particles to form. These clouds are the mine’s canary of global warming.
    The evidence linking carbon dioxide to changes in night-shining clouds is still tentative. But in all honesty, we don’t know of any other plausible reasons why it wouldn’t be global climate change. If that’s true, that says not only are we changing the atmosphere where we’re living, we’re changing a very remote, rarified part of our atmosphere. Such far-reaching change means that we need to be concerned. If the cloud changes are indeed tied to carbon dioxide build-up, here’s another confirming reason we should strive to do something about it.
    AIM also discovered "dramatic changes" in cloud behavior, said Scott Bailey, AIM’s deputy principal investigator. The data suggest the clouds are highly variable, rotating every five days according to minute shifts in temperature. " These clouds are exquisite thermometers. Now we’re able to see the whole life cycle of a cloud for the first time," Bailey told National Geographic.
What is implied in the sentence "These clouds are the mine’s canary of global warming. "(Lines 12-13, Paragraph 2)?
选项 A、Night-shining clouds may be a sign of global warming.
B、There is a canary in the mine below night-shining clouds.
C、Global warming is caused by night-shining clouds.
D、Night-shining clouds may accelerate global warming.
答案A
解析 第二段第十二至十三行的These clouds are the mine’s canary of global warming．暗示什么?[A]夜光云可能是全球变暖的一个信号。[B]夜光云下面的煤矿里有一只金丝雀。[C]全球变暖是由夜光云引起的。[D]夜光云可能会加速全球变暖。根据题干可定位在第二段最后一句。根据第二段可知：夜光云的变化可能是由气候变化引起的，故[A]的解释与原文最接近。[B]为字面解读，理解错误。在考试中，这类选项通常应首先排除。[c]和[D]颠倒了因果关系，故排除。答案为[A]。
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