That is the slogan-ized version of the conclusion of two recent studies on the affect of animal agriculture on global climate. This report from the UN Food and Agriculture Organization shows that "The livestock sector [is] responsible for 18 percent of all greenhouse gas emissions, measured in CO2 equivalent," which is more than all forms of transportation combined. And this conclusion is only one of several the UNFAO group draws showing the environmental damage of livestock. This study from geophysicists at the University of Chicago, and published in a journal called Earth Interactions, shows that "the greenhouse gas emissions of various diets vary by as much as the difference between owning an average sedan versus a sport-utility vehicle under typical driving conditions." That is, you could trade your Hummer for a Toyota, but it wouldn't do as much good as going vegetarian.
These studies are driving a campaign by major animal rights groups to link meat eating with global warming, highlighted in this NYT article (from Molly, via email.) I hope the campaign takes off. Past efforts to highlight this link generally get bogged down in jokes about cow farts. People have this immediate sense that if you are worried about cow farts, you must be some sort of moral busybody who wants to regulate everything. Cows are going to fart, how can you stop that? It helps to remember that the number of cows on the planet is not some pre-ordained fact. It is something we are responsible for. And hence, we are responsible for their mess.