I read somewhere that if you eat meat (I assume they were talking about beef) then its actually better for the environment co2 wise to just drive.
http://www.timesonline.co.uk/tol/news/uk/science/article2195...
But it was discussing walking, not riding (far more efficient), and used common milk as the calorie provision (effectively a worst-case scenario).
Please read this. Eating meat is the largest factor for green house gases. So if you want to go Green stop eating meat.