The New Scientist out of the UK has some interesting articles on eating meat and greenhouse gas emissions.
This first link is to a graph of what the average US family costs in GHGs due to food. What is important to see here is something I have known for sometime, transportation of food over long distances does not have a big impact. The methane is much more of a problem. The article is here.
Through that article I found this CO2 calculator for food - take it with a grain of salt because it is rather broad, but it is an interesting tool to look at.
The other interesting article is this analysis of what would happen if we backed off of beef and pork consumption. The article seems to say that by eating a low meat diet, we could reduce by 1/2 the costs of dealing with climate change by 2050. Low meat means 70 grams of beef a week and 325 grams of chicken or eggs. That is a burger every ten days and five eggs a week.
All in all, there is a lot more work being done on the greenhouse gas emissions that come from what and how we farm. It is becoming easier and easier to work out the impact of our diet. When I tried to work this out in the fall of 2007 there was not a lot of data I could access, but it was clear that meat eating was one of the biggest sources of greenhouse gas emissions in my life.