Is rapid climate change possible? What might cause it?

NASA: Yes. At the end of the last glacial period (10,000 years ago), the Younger Dryas event was a very sharp transition around the North Atlantic from warm conditions at the end of the last glacial period back to cold conditions which lasted about 1,000 years before switching back to warm conditions. Scientists think that this was caused by the last major ice sheet in North America collapsing and putting a lot of fresh water into the North Atlantic. This may have caused the North Atlantic "overturning" to slow down, reducing the amount of heat transported toward the pole. There was a smaller event that happened about 8,200 years ago, but that had a much shorter duration and the climate snapped back very quickly. No such events have occurred since then.

NSIDC: Temperature and salinity differences in the world's oceans work together to create what is known as thermohaline circulation. This circulation pattern, illustrated in the figure, acts as a "conveyor belt," slowly overturning the oceans. The strength of the thermohaline circulation is strongly determined by the sinking of the waters associated with the Gulf Stream. This releases vast amounts of heat to the atmosphere, keeping Eurpoe warm.

Readers may be interested in the following scientific articles:

• Alley, R., et al. 2003. Abrupt climate change. Science 299(5615): 2005-2010.
  
  
• Weaver, A. J., and C. Hilaire-Marcel. 2004. Global warming and the next ice age. Science 304 (5669): 400-402.