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Why the Earth’s surface temperature hasn’t warmed as expected over the past

游客2023-09-06  31
问题     Why the Earth’s surface temperature hasn’t warmed as expected over the past decade continues to be a puzzle for scientists. One study out earlier this month theorized that the Earth’s climate may be less sensitive to greenhouse gases than currently assumed.
    Another surprising factor could be the amount of water vapor way up in the stratosphere (平流层), according to a new study out Thursday in the journal Science.
    Water vapor, a powerful, natural greenhouse gas that absorbs sunlight and re-emits heat, is "a wild card" of global warming, says the paper’s lead author, senior scientist Susan Solomon of the National Oceanic and Atmospheric Administration.
    In the Science paper, Solomon and her colleagues found that a drop in the concentration of water vapor in the stratosphere "very likely made substantial contributions to the flattening of the global warming trend since about 2000".
    The decline in water vapor in the stratosphere slowed the rate of surface warming by about 25%, compared to that which would have occurred due to carbon dioxide and other greenhouse gases, notes the study. Specifically, the planet should have warmed 0.25 degree F during the 2000s, but because of the influence of the water vapor, it rose just 0.18 degree F.
    Why did the water vapor decrease? "We really don’t know," says Solomon, "We don’t have enough information yet."
    The findings are "surprising", says Bill Randel, an atmospheric chemist at the National Center for Atmospheric Research, who was not part of the study. He said it was surprising how big an effect such a very little change in stratospheric water vapor has had on the surface climate.
    These fluctuations in water vapor could be part of a feedback loop. Although it’s known that water vapor in the troposphere (对流层) increases as the climate warms — and is a major climate feedback that is well simulated in global climate models — in sharp contrast, models do a poor job of simulating water vapor in the stratosphere, according to the paper.
    But Solomon points out this isn’t an indication that predictions on global warming are overstated: "This doesn’t mean there isn’t global warming," notes Solomon. "There’s no significant debate that it is warmer now than it was 100 years ago, due to anthropogenic (人造的) greenhouse gases."
    And how will this water vapor affect future global warming? "We really don’t know the answer to this," says Solomon. "If the water changes are due to the specific way the sea-surface temperature pattern looks right now, then it may well not be linked to the overall warming. It could just be a source of variability from one decade to another as the ocean pattern slowly changes. Or it could be linked to the overall warming of the tropics, in which case it could continue to ’put the brakes on’. Only time will tell, and more data." [br] According to Solomon and her colleagues, how does water vapor affect global temperature?
选项 A、Global warming slowed down because water vapor in stratosphere dropped.
B、Global warming speeded up because water vapor in troposphere increased.
C、Global temperature lowered down because water vapor in troposphere dropped.
D、Global temperature rose up because water vapor in stratosphere increased.
答案 A
解析 根据题干中Solomon and her colleagues将本题出处定位到第四段。该段提到，Solomon和她的同事发现，平流层中水蒸汽浓度的下降“很可能对自从大约2000年起全球变暖趋势的减缓起到了实质性的作用”，也就是说，全球变暖的减缓是因为平流层中水蒸汽的下降，故答案为[A]。[B]是针对第八段中的water vapor in the troposphere increases as the climate warms设的干扰项，文中说的是随着气候变暖，对流层水蒸汽会相应增加，[B]是因果倒置，故排除，同时排除[D]“全球温度升高是因为平流层的水蒸汽的增加”。文中讲的是全球变暖的速度减缓，第九段明确讲到it is warmer now，故排除[C]。
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