“Researchers at the University of Chicago compared the global warming impact of meat eaters with that of vegetarians and found that the average American diet – including all food processing steps – results in the annual production of an extra 1.5 tons of CO2-equivalent (in the form of all greenhouse gases) compared to a no-meat diet. Researchers Gidon Eshel and Pamela Martin concluded that dietary changes could make more difference than trading in a standard sedan for a more efficient hybrid car, which reduces annual CO2 emissions by roughly one ton a year.”

http://www.csmonitor.com/2007/0220/p03s01-ussc.html

As the article indirectly points out, many of the environmental problems associated with meat wouldn’t really go away even if the animals were raised in a more humane fashion.

At the level of the individual, buying humanely-raised meat would certainly be an improvement from buying meat from factory farms. But what we should be concerned about when we talk about sustainable development, as I understand it, is the choices we would like society to make. And while humanely raised meat will always be an option for countercultural types–and let’s face it, anyone who gives much thought to these matters is going against the prevailing indifference of contemporary American culture–it cannot work for 300 million Americans. For one thing, where is the land to support enough livestock to feed so many people? For another thing, where are the farmers? If meat remains the order of the day, we have reached a point of no return; the factory farm will be our only option.

Hunting has the same problem. It can be practiced conscientiously by a few, but it cannot be relied on to provide for a large swathe of the population. (And then there’s the question of why you should kill an animal at all when you get all the nutrition you need from non-animal products.)