I'm quite sure that no one ever gave up meat-eating because they were harassed by environmentalists. (The opposite is probably more common.) Still, this study is striking: "A kilogram of beef is responsible for more greenhouse gas emissions and other pollution than driving for 3 hours while leaving all the lights on back home."
On the other hand, some beef is better than others: "A Swedish study in 2003 suggested that organic beef, raised on grass rather than concentrated feed, emits 40 per cent less greenhouse gases and consumes 85 per cent less energy." And the National Cattlemen's Beef Association wants you to put down that Boca burger and pay attention to everything they're doing to reduce methane emissions, thanks to "innovations in feeding practices."