During the termination of the latest ice age the warming climate leading from glacial to interglacial conditions was abruptly reverted by a distinct but short (about 1300 years in duration) cold event, referred to as the Younger Dryas. This event resulted in substantial moraine zones around the southern edges of the Fennoscandian ice sheet, indicating a halt in retreat. There are also evidence of strong permafrost conditions in the periglacial area. What caused this cold event?
The most accepted theory is that the Younger Dryas was caused by a large reduction or shutdown of the North Atlantic "Conveyor" because of a sudden influx of fresh water from Lake Agassiz and deglaciation in North America. (Although evidence for such an event is thus far lacking.) The global climate would then have become locked into this new state until freezing removed the fresh water "lid" from the north Atlantic Ocean.
Another theory suggests that the jet stream shifted northward in response to the changing topographic forcing of the melting North American ice sheet, which then brought more rain to the North Atlantic which freshened the ocean surface enough to slow the thermohaline circulation.
In addition to the theories mentioned by Azzie Rogers, there is another intriguing (but highly tentative) hypothesis: the YD may have been triggered by a large cosmic impact event. The results of a major study of impact-derived spherules at the time of the YD were recently published by Wittke et al. (2013). I should reiterate, though, that this is not currently a very well-supported hypothesis: the paper makes a fairly convincing case for the occurrence of some kind of impact, but mentions the possible climatic effect mainly as an avenue for future study:
The YD impact at 12.8 ka is coincidental with major environmental events, including abrupt cooling at the YD onset, major extinction of some end-Pleistocene megafauna, disappearance of Clovis cultural traditions, widespread biomass burning, and often, the deposition of dark, carbon-rich sediments (black mat). It is reasonable to hypothesize a relationship between these events and the YDB impact, although much work remains to understand the causal mechanisms.
So, definitely not a frontrunner for the YD trigger as yet, but one to keep an eye on if further evidence accumulates. The paper is open access, and makes for an interesting read.
- Wittke, J. H., Weaver, J. C., Bunch, T. E., Kennett, J. P., Kennett, D. J., Moore, A. M., ... & Firestone, R. B. (2013). Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 y ago. Proceedings of the National Academy of Sciences, 110(23), E2088-E2097. DOI: 10.1073/pnas.1301760110