The Younger Dryas was a cooling event about 12 kyr ago – see the wikipedia article, which still has the pic I drew in it. It is an example of rapid climate change, and was probably caused by THC shutdown caused by meltwater from the Laurentide icesheet, though exactly how is unclear.
However, some people would like it to be caused by a comet. That was first suggested in 2007, and RC said we doubt it. Then again in 2009 some more evidence was produced, and RC said we still doubt it. Others were less cautious; and Unscientifc American gushed over it as did Nat. Geog.. Bits of their work turned out to be non-reproducible, and it also fails various subsidiary tests. See-also Skeptical Inquirer â¢ May/June 2010 by David Morrison, which also notes the dubious pseudo-science precursors of the idea.
More excitingly, now Comet Theory Comes Crashing to Earth says the entire theory is nonsense, and possibly fraudulent too. I know nowt of the “Kennett” mentioned in that article, but he can’t be famous (well, because I’ve never heard of him, and) because he has no wikipedia article. It looks a bit like K-the-elder (confusingly, there seem to be two of them, one the son of the other) is the influential crusty old type gone Emeritus who is getting more respect for his theory than it deserves. Or then again, he may have been traduced*. But the villain may be West, who is really called Whitt, if my source is to believed. If the new article is right (or if only the Skeptical Inquirer is right) then the motto is the usual one: don’t believe a sexy new idea just because it appears in a high-profile journal.
* Kafka joke.