Most predictions of global climate change catastrophe feel that what's likely to happen is that Greenland will melt, dumping huge amounts of water into the Atlantic. I cannot help but believe that that water will most likely be only slightly warmer than freezing, and therefore couldn't do anything but cool the oceans. On the other hand, that water is likely to be freshwater, and therefore potentially warmer than the saltwater it would be flowing into, so I could be wrong.