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  3. 　　In their darker moments, climatologists talk about their own "nightmare scenario". This is one where global warming has caused

　　In their darker moments, climatologists talk about their own "nightmare scenario". This is one where global warming has caused

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问题 　　In their darker moments, climatologists talk about their own "nightmare scenario". This is one where global warming has caused such significant climatic changes that ocean currents change direction. One scene from the nightmare has the Gulf Stream moving south or even going into reverse, making winter in London look and feel like a St Petersburg January.
　　The ocean is a great moderating influence on the planet, soaking up heat around the tropics and depositing it in the cooler polar regions. Yet scientists know surprisingly little about how the sea does this―they estimate that the North Atlantic alone moves energy equivalent to the output of several hundred million power stations.
　　Last year oceanographers began their biggest international research initiative to learn more about ocean circulation. The first results from the World Ocean Circulation Experiment demonstrate just how complex the movement of sea-water can be. They have also given scientists a glance of the amount of heat being exchanged between the oceans and the atmosphere. As part of the experiment, researchers are monitoring the speed and direction of ocean currents, water temperature and salinity.
　　Research ships taking part will gather detailed measurements at 24,000 points or "stations" a long carefully designated trans-ocean mutes. This undertaking dwarfs the 8,000 hydrographic stations created in the past hundred years of ocean surveying, A fleet of ships, buoys, seabed sensors and satellites will collect so much data that Britain, one of the 40 countries taking part, has opened a research institute, the James Rennell Centre for Ocean Circulation in Southampton, to process them.
　　One of the justifications for the experiment, says John Woods, director of marine and atmospheric sciences at the Natural Environment Research Council, is that the oceans hold the key to understanding long-term changes in the global climate. The Earth has two "envelopes"―the ocean, consisting of slowly circulating water, and the atmosphere, made of fast-moving air. Far from being independent, they interact, one modifying the other until a balance is reached between them. The present balance came about at the end of the last Ice Age, about 10,000 years ago. Scientists hope that knowing more about the ocean’s "weather patterns" will help them to predict climate changes further ahead.
　　Knowing how heat is moving around the ocean is decisive to such long-term forecasting. The top three metres of the ocean store more heat than all of the atmosphere. Some of the heat can be transported downward between 30 metres and several thousand metres. The deeper it goes, the longer it stays out of the atmosphere. Water heated in the equatorial region flows in shallow currents north or south towards the poles, where it releases its heat to the air and, as it becomes colder and denser, sinks to the sea floor, where it forms deep, cold currents that back to the equator.
　　John Gould, one of the British scientists taking part in the ocean circulation experiment, is discovering just how this occurs in the North Atlantic. Shallow currents, less than 500m deep, of warm water at about 8℃ flow from the Atlantic into the Norwegian Sea, mainly along a path that follows the point where the continental shelf ends and the deep mid-ocean valleys begin. Meanwhile, at depth down to 5,000, deep currents of cold water at about minus 1℃ flow south into the Atlantic along the deep ocean valley. (Salt water at this depth does not freeze at 0℃)
　　Sensors positioned on the seabed have given Dr. Gould and his researchers an accurate assessment of just how much cold water is flowing back into the North Atlantic and have given up its heat to the atmosphere over north-west Europe. In total, he estimates, about 5 million cubic metres of water per second flows in these deep currents between Greenland and the British Isles. This means the warm water of the North Atlantic must be giving up about 200 million megawatts of energy to the atmosphere over north-west Europe.
　　Research at the other end of the world, in the seas around Antarctica, is also finding that sea floor topography plays a crucial role in determining the direction of ocean currents. In the past, oceanographers have assumed, for instance, that surface currents such as the Gulf Stream do not extend much beyond a kilometre in depth. But an analysis of currents in Antarctic waters has shown that cur rents are not concentrated in the top kilometre, but reach down to the submerged mountain ranges.
　　Dr. Woods believes such research will help to save lives. "More deaths can be prevented by ocean forecasting, than by weather forecasting and our economic and social well-being are more vulnerable to change in the ocean than in the atmosphere".
选项 A、modify ocean currents.
B、change wind directions in the polar regions.
C、reduce the influence of currents.
D、increase wind speed.
答案A
解析 文中第一段提到"This is one where global warming has caused such significant climatic changes that ocean currents change direction"。
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