Climate is hard to predict. This sudden cooling has no known cause:-"Scientists agree that the brief episode at the end of the Ice Age -- officially known as the Younger Dryas for a flower that flourished at that time -- sparked widespread cooling of Earth 12,800 years ago and that this cool period lasted for 1,000 years. But theories about the cause of this abrupt climate change are numerous. They range from changes in ocean circulation patterns caused by glacial meltwater entering the ocean to the cosmic-impact theory."-http://www.sciencedaily.com/releases/2014/05/140513113605.htm