grow dense and sink to great depths and begin a slow trip back to the southern hemisphere. 密度增加，沉到深处，慢慢返回南半球。

　　When they reach Antarctica, they are caught up in the Antarctic Circumpolar Current, where they are driven onward into the Pacific. 这批海水抵达南极洲，遇上了南极绕极流，被往前推入了太平洋。

　　The process is very slow—it can take 1,500 years for water to travel from the North Atlantic to the mid-Pacific, 这个过程是很慢的——海水从北大西洋流到太平洋中部要花1500年时间，

　　but the volumes of heat and water they move are very considerable, and the influence on the climate is enormous. 但它运送的热量和水量是相当可观的，对气候的影响也是巨大的。

　　(As for the question of how anyone could possibly figure out how long it takes a drop of water to get from one ocean to another, （怎么有人可能计算出一滴水从一个大样到另一个大洋要花多长时间？

　　the answer is that scientists can measure compounds in the water like chlorofluorocarbons and work out how long it has been since they were last in the air. 关于这个问题，答案是：科学家们可以测定水里的混合物，比如含氯氟烃，从而计算出自它上次进入空气以来已有多长时间。

　　By comparing a lot of measurements from different depths and locations they can reasonably chart the water's movement.) 通过把不同深度、不同地点的测量结果进行比较，可以比较准确地绘制出水的运动路线图。）

　　Thermohaline circulation not only moves heat around, but also helps to stir up nutrients as the currents rise and fall, 热盐对流不仅传送热量，而且随着洋流的起伏，也起着搅动食物的作用，

　　making greater volumes of the ocean habitable for fish and other marine creatures. 使更大范围的海域适于鱼类和其他海洋动物生存。

　　Unfortunately, it appears the circulation may also be very sensitive to change. 不幸的是，热盐对流对周围的变化似乎也很敏感。

　　According to computer simulations, even a modest dilution of the ocean's salt content, 计算机模拟试验的结果表明，连海洋盐度稍有稀释，

　　from increased melting of the Greenland ice sheet, for instance—could disrupt the cycle disastrously. 比如，由于格陵兰冰原的加快融化，也会灾难性地打乱这个循环。

　　The seas do one other great favor for us.  大海还帮了我们另一个大忙。

　　They soak up tremendous volumes of carbon and provide a means for it to be safely locked away. 它吸收大量的碳，并有办法把碳藏到安全的地方。