It's not necessarily the disruption to the food chain itself from a temperature change of only a few degrees--warmer temperatures reduce crop yields quickly, which in turn affects meat prices as grain-fed animals become much more expensive. Major increases in the price of food won't necessarily cripple America, but have the potential to cause devastating mass starvation in the third world.

http://www.economist.com/node/18386161
http://phys.org/news/2013-02-climate...orn-crops.html