Cow farts are carbon neutral. Grass sucks the carbon out of the atmosphere, cows turn carbon into dinner and flatulence, carbon re-enters the atmosphere. Get rid of cows, and you'll have grass fires returning that carbon to the atmosphere.
I can't believe I'm seriously posting about cow farts. Must get back to work. Must. get. back. to. work.
Eating less meat will make a bigger impact than driving less.