Jayson Lusk has a nice post: How bad, really, is eating meat. He takes some EPA based carbon footprint/externality/social cost calculations and applies them to the price of meat. Basically the value of the impact of meat on the climate amounts to a few cents per serving.
You don't have to be a climate change denier to deny the embellished drama of most climate change policy advocates....beef is not that bad when it comes to carbon footprint and it's getting better....and remember the numbers are not as good for local/grass finished/hormone free vs. conventional but I would say still have an overall low impact b/c that market is small.
See also: Doing Nothing: A science based policy prescription for climate mitigation