Perhaps the most curious aspect of Earth's emergence from the last glacial maximum, a process that began just under 20,000 calendar years ago, was the Younger Dryas, (named after the pollen-record of the beautiful plant that is now grown in many rock-gardens - my Grandfather grew it and I did in my time): a period beginning 12,900 years before present and lasting over 1,000 years during which the planet, and especially the northern hemisphere, cooled rather abruptly and, at high latitudes, glacial re-advance commenced.

Ending even more abruptly, perhaps in as little as a few decades, it marked the final cold snap before the advent of the more temperate conditions of the Holocene Period.

So: what caused it?

Source: Skeptical Science (http://s.tt/1805b)