It is not being caused by the CO2 in the atmosphere, stupid.

Comet Killed Ice Age Beasts

The longer-term effects would have killed most of the lingering survivors.

"An explosion like this of a comet injects a lot of water vapor (comets are made of water ice) so it basically would've put clouds over most of the Northern Hemisphere," West said. "That's going to make things get very cool very quickly."

http://www.foxnews.com/story/0,2933,534137,00.html