In a report titled The Cheeseburger Footprint, Jamais Cascio estimates that the average cheeseburger generates between 6.3 to 6.8 pounds of CO2 emissions. He references Fast Food Nation, among other sources, to approximate the number of cheeseburgers consumed per American annually- roughly 150. Multiply that by a population 300,000,000, and the resulting collective carbon footprint of the American appetite for cheeseburgers is (conservatively) 195,750,000 metric tons of CO2 equivalent (remember cows create methane which is a far more potent greenhouse gas than CO2).
Cascio goes on to calculate the global warming impact of driving SUVs in comparison to eating cheeseburgers. He concludes, "the greenhouse gas emissions arising every year from the production and consumption of cheeseburgers is roughly the amount emitted by 6.5 million to 19.6 million SUVs. There are now approximately 16 million SUVs currently on the road in the US."
Here's some food for thought: all things considered, our food choices may be as important, if not more, than our transportation choices when it comes to climate change.