Prof. Wally Broecker is an extremely prolific scientist who first came up with the term "Global Warming" and who first described many of the global oceanic processes that control climate variability on the earth. In 1987 Prof. Broecker wrote a scientific paper entitled "Unpleasant Surprises in the Greenhouse" that introduced the idea that continued global warming might result in unexpected abrupt shifts in global climate, rather than a slow steady progressive warming.
One of Prof. Broecker's greatest concerns was that the thermohaline circulation system (also known as the global conveyer belt) might break down. The thermohaline circulation system is a global system of both surface and submarine oceanic currents that begins when cold, dense ocean water near Greenland and Antarctic sinks into the ocean and gives rise to deep ocean currents that travel from the poles to the equator. These deep ocean currents are connected by upwelling to warm surface currents like the Gulf Stream, creating a global ocean circulation system.
Broecker hypothesized that global warming might reduce the amount of deep ocean water being produced off Greenland, weakening and perhaps even stopping the thermohaline circulation system. This could produce large abrupt climate changes in northern Europe and elsewhere. The science fiction move "The Day After" was very loosely based on this idea, although the effects of a thermohaline shut down were greatly exaggerated.
Yesterday I wrote about the record warm temperatures seen during the winter of 2014-15. While almost the entire world was seeing above average temperatures, parts of the north Atlantic Ocean just south of Greenland are curiously colder then average. Why is this happening---why is this particular region cooling down when almost the entire rest of the planet is warming?
Some scientists are suggesting that the curious cooling seen south of Greenland is a clear sign that the global thermohaline circulation system is weakening right now. That doesn't mean that the global thermohaline circulation system is going to stop today, but it does raise concerns that it might stop soon---perhaps even the day after tomorrow.
W.S. Broecker, “Unpleasant surprises in the greenhouse?”, Nature, vol. 328, pp. 123-126, 1987. http://dx.doi.org/10.1038/328123a0