From Audobon Magazine:
Simply put, raising beef, pigs, sheep, chicken, and eggs is very, very energy intensive. More than half of all the grains grown in America actually go to feed animals, not people, says the World Resources Institute. That means a huge fraction of the petroleum-based herbicides, pesticides, and fertilizers applied to grains, plus staggering percentages of all agricultural land and water use, are put in the service of livestock. Stop eating animals and you use dramatically less fossil fuels, as much as 250 gallons less oil per year for vegans, says Cornell University’s David Pimentel, and 160 gallons less for egg-and-cheese-eating vegetarians.
But fossil fuel combustion is just part of the climate–diet equation.