Also, according to the UN food reports, we produce enough non-meat to feed the world population twice over. If we took meat production out of the equasion (which I vaguely recall consumes some of that production, as does biofuel programs), I would imagine that we would be growing less crops overall due to a decrease in overall demand. Depends on how the transition (from using crops to feed livestock --> not using crops to feed livestock) played out.
Methane gas from a cows bottom takes a rather long time for the atmosphere to process out, where carbon dioxide takes very little. There is also the highly inefficient transportation of livestock to slaugher, then from slaughter to packing (this step is typically necessary for hygine purposes but some slaughter plants also cut and pack), and packing to store. Veggies can go from farmers field (with pre-inspection) to farmers market or super market. In this case can does not necessarily mean should, but it is noteworthy. Or, personal garden to dinner table.As for Serpentine's point, I once again never said everyone everywhere should stop eating meat entirely, and fully recognize it can be a sustainable practice. That doesn't change the fact that the meat industry is one of the leading causes behind deforestation and has a huge impact on greenhouse gasses.
Then there is the runoff from high density feedlots and slaugher plants. It's been blamed on... well quite a few things. And it uses up copious amounts of water.
Environmental reasons are probably the most noteworthy arguement for reduction of meat servings in one's diet. The moral arguement is not really objective, the diet arguement is kind of a gray area, but the environmental arguement is pretty solid. The harm caused is noticeable, but it depends entirely on the source.
I'm a moderate sort of person, hence why I think that cutting back to sustainable-only would solve most but not all of the issue. At worst it gets rid of the environmental arguement, and leaves the moral and diet arguements which largely boil down to personal choices.