Basically, there was a climate study done for the UN that found that the impact caused by the consumption and production of meat in the world was greater than that caused by ALL transportation exhaust. I think it's interesting, though, that the climate chief delivering these results is vegetarian. Conflict of interest or perspicacity on his part? I'll let you decide.
But, as a vegan cyclist, all I've got to say is, "Nyah!"
Here's a link to an article from the BBC on the subject.