The deep-sea record shows evidence of abrupt climatic change centered on the last deglaciation (14-8 ka), and resulting in a severe cold spell between 11,000 and 10,000 years ago, known as the Younger Dryas period. The origin of this climate catastrophe is not known. Three types of possible causes must be considered: (1) running the system's positive feedback loop in reverse (albedo, CO 2, ocean circulation), (2) disturbance from internal threshold feedback (collapse of ice sheets), and (3) system-external forcing (volcanism, solar output, supernova, cosmic dust). A shutdown of the Nordic heat pump due to excessive meltwater input is one possible cause for the cold spell. This pump is driven by North Atlantic Deep-Water (NADW) production and results in advection of warm water to the Norwegian-Greenland Sea. However, NADW shutdown or slowdown occurred both before and after the Younger Dryas, while warming proceeded rapidly. Consequently, other heat pumping mechanisms may have been more important during deglaciation (e.g., import of warm surface or intermediate waters and export of cold surface water and floating ice). It was the interference with those mechanisms, then, which aided in the Big Freeze. In addition, a short-term reduction in pCO 2 during the Younger Dryas appears indicated. The search for a specific cause for the Younger Dryas cold spell may be futile. In a chaotic system near its point of bifurcation, small disturbances can result in large effects, from positive feedback amplification. Every link in the feedback loop is both cause and effect. The possibility of external influence cannot be discounted: there is evidence both for increased volcanic activity and extraterrestrial disturbance at the time.