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  3. Global warming could actually chill down North America within just a few decades, according to a new study that says a sudden co

Global warming could actually chill down North America within just a few decades, according to a new study that says a sudden co

admin2015-10-21  69
问题     Global warming could actually chill down North America within just a few decades, according to a new study that says a sudden cooling event gripped the region about 8,300 years ago.
    Analysis of ancient moss from Newfoundland, Canada, links an injection of freshwater from a burst glacial lake to a rapid drop in air temperatures by a few degrees Celsius along North America’s East Coast. This event created a colder year-round climate with a much shorter growing season for about 150 years, from northern Canada to what is now Cape Hatteras, North Carolina. The results suggest that North America’s climate is highly sensitive to meltwater flowing into the ocean, said lead study author Tim Daley of Swansea University in the U. K. The work also means that history could repeat itself: Currently Greenland’s ice sheet is melting at a rapid clip, releasing freshwater into the North Atlantic. In a worst-case scenario, the authors say, a sudden melt could trigger another regional cooling event—although other experts say today’s extreme, human-driven warming might cancel out any strong cooling effect.
    Daley and colleagues studied mosses dating back more than 8,700 years that were preserved in a Newfoundland peat bog. The ratios of two different types of oxygen in the mosses allowed the team to trace changes in atmospheric temperature over time. When air temperatures are lower, the mosses contain less oxygen-18, a heavier version of the more common type, oxygen-16. About 8,350 years ago, the amount of oxygen-18 relative to oxygen-16 suddenly dropped, the team reports in the September issue of the journal Geology. Previous research had found that, around the same time, a northern ice dam burst, releasing the contents of a vast glacial lake into the Labrador Sea, between Canada and Denmark. Normally a warm ocean current called the Gulf Stream runs up the east coast of North America, helping to keep the region balmier than it should be, considering how far north it is. But the entire glacial lake drained within less than a year, injecting a huge pulse of freshwater into the North Atlantic Ocean. Daley and colleagues think the lake water diluted the salty ocean current and slowed the Gulf Stream, which in turn led to rapid cooling in North America. "As a result, Canadian summer temperatures would have been similar to those currently experienced in autumn or spring," said team member Neil Loader, also of Swansea University. Climate records from Greenland and Europe also show a sudden cooling during the same time period, but this is the first clear evidence for a North American chill.
    The moss data show that current climate models " significantly underestimate the impact and duration of the climate perturbation resulting from the megaflood," said Swansea team member Alayne Perrott. This means these same models might not be accurately predicting what might happen in the future if Greenland’s ice sheet continues to melt. However, some scientists say that the data showing a prehistoric North American cool down may only indicate a coastal phenomenon.
    "The study site is very close to the North Atlantic Ocean, and it is very likely that the climate change is primarily an oceanic signal," said Hans Renssen, a climate researcher at Vrije University in Amsterdam, who was not involved in the study. As for whether today’s melt in Greenland could trigger another round of cooling, Renssen thinks it’s possible, but he doesn’t believe the change would be as dramatic as last time. In fact, he said, any future cooling is likely to be overwhelmed by human-caused warming, "resulting in no cooling in North America at all, only less warming than without the event. "  
Which of the following best describes the author’s development of argument?
选项 A、Introducing the issue→citing ways to deal with the issue→describing the actual status→ offering the reasons.
B、Introducing the issue →providing the evidence→furthering the research→listing all the possible explanations.
C、Introducing the issue→bringing forward the contrast ideas→comparing the differences→ making the final conclusion.
D、Introducing the issue→finding supportive examples→applying the solution to the problem →telling the final result.
答案B
解析 篇章题。本题考查对文章整体内容的把握。本文围绕地球究竟是会变暖还是会变冷展开讨论。第一段点明主题，指出有科学家研究，地球可能会因为冰壳的融化而变冷——“introducing theissue”；第二段给出研究的具体情况——“providing the evidence”；第三段进一步对研究的情况进行说明——“furthering the research”；第四、五段中却又提出了其他科学家的观点——“listing all thepossible explanations”。因此[B]为答案。
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