evidence: On our watery planet the climate is governed largely by the oceans. But for a long time, discussions of climate change did not take the oceans fully into account, simply because very little was known about them. It seemed that the tremendous masses of water could vary only over a geological timescale. Ideas began to change at the start of the 1960s. Studies of clay extracted from the seabed, supported by new theoretical ideas, suggested that ocean current patterns might shift within mere thousands of years. Other studies began to build a picture of the complex and surprisingly fragile circulation of the world-ocean. In the 1980s, evidence from Greenland ice cores, supported by crude computer models, showed that the North Atlantic circulation could come to an abrupt halt within the space of a century or two. Fears arose that global warming might trigger such a switch, which could wreak serious harm. Improved models calculated that this was unlikely to happen in the 21st century, although some slowing was observed. What was much more certain was that the oceans were rapidly warming and growing more acidic, exactly as predicted by the greenhouse gas theory.