Ancient flood

http://environment.newscientist.com/article/dn113-ancient-flood-brought-gulf-stream-to-a-halt.html

About 8000 years ago, a huge glacial lake in Canada burst, and the fresh water rushed into the North Atlantic. Researchers now say they know for sure that this catastrophic event shut down the Gulf Stream and cooled parts of the northern hemisphere by several degrees for more than a hundred years.