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  3. It was a bad sign if subtle shift in the far North Atlantic. For 30 years, waters off southern Greenland and Iceland had been gr

It was a bad sign if subtle shift in the far North Atlantic. For 30 years, waters off southern Greenland and Iceland had been gr

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问题     It was a bad sign if subtle shift in the far North Atlantic. For 30 years, waters off southern Greenland and Iceland had been growing less and less salty, oceanographers reported in late 2003. It looked as if global warming could be freshening high-latitude Atlantic waters. lf the trend continued, they worried, it could throw a monkey wrench into the "conveyor belt" of currents that warms the far North Atlantic, as is wildly overdone in the movie The Day After Tomorrow. New analyses have now shown that global warming is indeed messing with the Atlantic’s salinity, although not as dramatically as Hollywood envisioned.
    The first explicit link between global warming and ocean salinity changes comes in a study in press in Geophysical Research Letters. Modeler Peter Scott of the Met Office Hadley Center in Exeter, U. K. , and his colleagues simulated changing ocean salinity in the center’s HadCM3 climate model with and without increasing greenhouse gases. Under past global warming conditions, the model produced salinity changes around the world much like those seen. But only in the subtropics and mid-latitudes of the North Atlantic—between 20° N and 50° N—did salinity change significantly more than the natural jiggling of the climate system would have changed it.
    The increase in salinity in North Atlantic mid-latitudes thus carries the "fingerprint" of human influence left by the effects of human-generated greenhouse gases, the group concludes. Greenhouse warming there apparently removed more water by evaporation while precipitation decreased, concentrating seawater’s salts. "It looks convincing to me", says climate modeler Gabriele Hegerl of the University of Edinburgh, U. K. The signal is only now emerging, she adds, but "it looks very consistent with what is expected. " She and others would like to see additional models replicate the fingerprinting, just to be sure.
    Although global warming seems able to alter even the saltiness of the sea, it hasn’t noticeably freshened the high latitudes of the North Atlantic, as some researchers thought it might be doing back in 2003. When run without rising greenhouse gases, the Hadley model produces some many salinity swings up and down through natural processes built into the climate system that any greenhouse fingerprint would have been smudged beyond recognition, the group found.
    But global warming isn’t finished with the far North Atlantic, at least according to the Hadley model. Run out to the end of the century under a strengthening greenhouse, it simulates a precipitous dip in northern salinity from a recent upturn and then a rapid recovery by 2100. That roller-coaster ride "rings true," says physical oceanographer Ruth Curry of Woods Hole Oceano-graphic Institution in Massachusetts. Since publishing the observed freshening trend in 2003, she has come to understand that natural swings in atmospheric circulation over the North Atlantic—the so-called North Atlantic Oscillation(NAO)—have alternately driven fresher water from the Arctic and then saltier water from low latitudes into the far North Atlantic. Those shifts, rather than global warming, have dominated high-latitude salinity, with an NAO-driven switch from fresher to saltier coming in the mid-1990s.
What does the passage mainly talk about?
选项 A、Greenhouse and global warming.
B、North Atlantic is changing.
C、Global warming and the salinity of North Atlantic.
D、NAO and the salinity of North Atlantic.
答案C
解析 主旨大意题。通读全文可知，全文通篇都在讨论温室效应和海水盐度之间的联系。[C]与之相符，故正确。[A]、[B]、[D]均不准确。
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