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  3. The climate of earth has fluctuated quite a bit over the last 4.6 billion years of our planers history and it can be expected th

The climate of earth has fluctuated quite a bit over the last 4.6 billion years of our planers history and it can be expected th

admin2013-06-02  29
问题      The climate of earth has fluctuated quite a bit over the last 4.6 billion years of our planers history and it can be expected that the climate will continue to change. One of the most intriguing questions in earth science is whether the periods of ice ages are over or are we living in an "interglacial" period of time between ice ages?
     The geologic time period we are now living in is known as the Holocene(全新世). This epoch began about 11,000 years ago which was the end of the last glacial period and the end of the Pleistocene (更新世) epoch. The Pleistocene was an epoch of cool glacial and warmer interglacial periods which began about 1.8 million years ago.
     Since the glacial period known as the "Wisconsin" in North America and "Wrm" in Europe when over 10 million square miles (about 27 million km2) of North America, Asia, and Europe were covered by ice, almost all of the ice sheets covering the land and glaciers in the mountains have retreated. Today, about ten percent of the earth’s surface is covered by ice; 96% of this ice is located in Antarctica and Greenland. Glacial ice is also present in such diverse places as Alaska, Canada, New Zealand, Asia, and California.
     As only 11,000 years has passed since the last Ice Age, scientists can not be certain that we are indeed living in a postglacial Holocene epoch instead of an interglacial period of the Pleistocene and thus due for another ice age in the geologic future. Some scientists believe that an increase in global temperature, as we are now experiencing, could be a sign of an impending ice age and could actually increase the amount of ice on the earth’s surface.
      The cold, dry air above the Arctic and Antarctica carries little moisture and drops little snow on the regions. An increase in global temperature could increase the amount of moisture in the air and increase the amount of snowfall. After years of more snowfall than melting, the Polar Regions could accumulate more ice.
    An accumulation of ice would lead to a lowering of the level of the oceans and there would be further, unanticipated changes in the global climate system as well. Our short history on earth and our shorter record of the climate keeps us from fully understanding the implications of global warming. Without a doubt, an increase in the earth’s temperature will have major consequences for all life on this planet.  
According to the passage Holocene ______.
选项 A、ended about 11,000 years ago
B、is an existing geologic time period
C、is the beginning of the Pleistocene epoch
D、is an epoch of cool glacial and warmer interglacial periods
答案B
解析 细节题。由文章第二段第一句可知，B正确。
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