Hey, I was trained as an agricultural economist and I work in sustainable agriculture now. There is an increasing awareness that vegetarianism is NOT a panacea for climate change and that those who initially pushed it as such did not know much about the realities of agriculture. The reality, which is portrayed pretty well in the aforementioned Lierre Keith book, is that grain agriculture is incredibly unsustainable and destructive. Grain agriculture is what caused the huge "dead zone" in the Gulf of Mexico. Vegetarians might point out that some of those grains go to feed livestock, but that doesn't change the fact that the vegetarian diet doesn't do much to wean us from industrial monoculture. I recently heard Wes Jackson, a prominent plant geneticist, speak and he said that currently, the agricultural system we have is point blank unsustainable and destructive whether we are eating whole wheat bread or chicken mcnuggets.
A paleolithic diet based on local grassfed meat is one of the few that is truly sustainable. That brings me to another vegetarian myth, which is that if we stopped eating meat we could feed the world. Sorry, we already produce enough food to feed the entire world- problems with hunger are problems due to distribution and politics. Humans can't eat what pastured livestock can: compost in the case of pigs, grass in the case of cows, brushy weeds in the case of goats. And if you tried to grow grains on pasture you would destroy the grass-based ecosystem, leading to soil runoff and ecological destruction. As far as methane emissions, the report that said that 50% of global warming gases are caused by livestock is laughed at by economists...it makes NO sense and was a hack job.
Methane can be an issue though, so if you are really concerned with that, chose the most sustainable meat: pork. Pigs are fed compost which other animals can't eat and they are not belching ruminants.On a sustainable farm their waste doesn't go in fermentative lagoons, but to fertilize crops. Oh, BTW, let's talk about fertilizer. You can't grow much of anything without fertilizer. Right now, they only respectable alternative to animal waste is oil-based....how is that more sustainable? Unfortunately, right now the waste from factory farms really is doing damage to the environment. Farms aren't integrated the way they used to be, partially because of bad regulations, and instead of fertilizing crops, the manure just sits around polluting the environment. Thankfully, you can chose to get your meat from a small integrated farm...either from a farmer's market or a CSA. Look at localharvest.org to find one near you.
I also think it's frankly laughable and irresponsible that vegan groups like HSUS are advocating a vegetarian diet as a cure for climate change. The effects would be negligible and perhaps even negative given that most people switching would probably go for processed imported trash like Boca burgers. Colin Beavan, No Impact Man, eats a diet of local beans and vegetables, which I will concede might have less impact, but it's so much more complex than meat vs. vegetables. It depends on how destructive to the soil growing these things was...rotational grazing is usually waaaaaaay less destructive.
That said, since I eat a diet based around grassfed local meat, I do eat less meat than people would expect. But that brings me to another way paleo is more sustainable...we don't shun the fat! God knows how many calories goes to waste from people trimming fat. I use ALL my fat and it allows me to eat probably less meat than the average American, but get plenty of calories and fat soluble vitamins from it.