I've posted on this before, but looks like the report on comet impact at 12,900 years ago has just been published in the Journal "Nature". The link is to a news article on this, not the actual article. I had heard (from one of the members of the group working on this) that the impacts had de-stabilized the north american ice sheet. I did not (and do not) understand how that might have worked. The linked news story says that heat from the impacts melted the ice sheet (which was already melting), but this somehow seems unlikely...although I suppose if a large comet hit right on it, some part would be vaporized etc. The idea that the Younger Dryas cooling was related to melt water disrupting ocean circulation has been around for a decade or more.
Chris

• news story on Nature impact article