Abrupt Climate Change
For those of you unfamiliar with the abrupt climate change idea, some of the most informative pieces about what it could entail can be found at the Woods Hole Oceanographic Institution website. In (extreme) summary, there is a warm salt water flow in the Atlantic Ocean which brings warm water -- and, by extension, warm weather -- to northern Europe (this is why Paris, which is at a latitude roughly equivalent to Winnipeg, Canada, has weather much closer to that of San Francisco). Global warming melts Arctic glaciers and icebergs, dumping very cold fresh water at the north end of the salt water flow; a sufficient amount will cut it off, thereby cutting off the warm temperature flow to northern Europe. If that happens, in a startlingly brief amount of time -- perhaps as few as two or three years, and likely no more than a decade or two -- northern Europe would be plunged into Siberian-like conditions, a mini-ice age which could last centuries. It's unclear what would happen in Asia and North America; the mini-ice age could be generalized across the entire hemisphere, or the heat from the ocean flow could be shifted, leading to a paradox climate of Siberian Paris and Tropical Chicago.
We know that this is plausible because it has happened before. Ice core samples from Greenland can be used to measure annual temperature changes over tens of thousands of years. Repeatedly, very quick drops into ice age conditions were preceded by warming periods. And it could well happen again, and soon; over the last couple of decades, average North Atlantic salinity has dropped precipitously.