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  3. Take a minute to familiarize yourself with the Atlantic Meridional Overturning Circulation (AMOC). 【F1】It’s a massive ocean curr

Take a minute to familiarize yourself with the Atlantic Meridional Overturning Circulation (AMOC). 【F1】It’s a massive ocean curr

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问题    Take a minute to familiarize yourself with the Atlantic Meridional Overturning Circulation (AMOC). 【F1】It’s a massive ocean current system that’s critical to the stability of our climate, as it moves a significant amount of heat from the Southern Hemisphere and the tropics up toward the North Atlantic, where that heat is released to the atmosphere. For Europe and North America, the AMOC helps regulate how seasons are experienced. But in a study published Friday in the journal Nature Communications, scientists at the University of South Florida (USF) used NASA data to conclude that Greenland’s rapidly melting ice sheet could be slowing down the AMOC, (according to a map of the surface currents (solid curves) and deep currents (dashed curves) that form a portion of the Atlantic meridional overturning circulation). And that could be a problem.
   【F2】As the Greenland ice sheet melts and the meltwater runs off (and it’s running off much faster than previously thought according to the latest research), more and more cold freshwater is pushed into the Labrador Sea. The scientists used data from NASA’s gravity-sensing GRACE satellite mission to determine that freshwater is significantly "freshening" the North Atlantic Ocean. Freshwater is more buoyant than salt water, and the GRACE mission can detect that subtle difference in height—and therefore gravitational pull—of the more-buoyant water.
   【F3】The Greenland runoff, then, "has the potential to increase the buoyancy of surface waters and reduce formation of dense, deep water that helps drive the overturning circulation," explained Don Chambers, a professor of marine science at USF College and an author on the study, in a statement. That, in turn, could "weaken," or slow down, the AMOC. In other words, this ocean current could be knocked off-kilter. "The major effect of a slowing AMOC is expected to be cooler winters and summers around the North Atlantic, and small regional increases in sea level on the North American coast," Chambers said.
   【F4】On a global scale, however, the effects are much harder to understand—but could be more consequential. "The AMOC and Gulf Stream are part of a complex global ocean circulation system that is still not completely understood," Tom Dixon, another USF professor and author on the study, said. "【F5】If human activities are starting to impact this system, it is a worrying sign that the scale of human impacts on the climate system may be reaching a critical point."
【F5】
选项
答案如果人类活动已经开始影响这一系统，那么让人担忧的迹象是，人类对气候系统的影响程度可能已经达到了临界点。
解析 ①本句为较复杂的主从复合句，带有一个条件状语从句，主句中带有that引导的主语从句。②主句中的it是形式主语，真正的主语为that引导的主语从句that the scale of．．．on the climate system…a critical point。介词短语on the climate system作后置定语，修饰the scale of human impacts，说明人类影响规模的对象。impact on…意为“对……的影响”。
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