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Regarding the sustainability of eating meat. It's worth remembering that not all land that is currently used for animal rearing and grazing is feasible arable land. For example hillside that can be grazed by sheep might be completely unsuitable for growing anything that humans can eat. It doesn't therefore have to cost any land to grass-feed an animal. Likewise, if you feed your own pig only on scraps that you can't eat yourself, you're generating food (the pig) at no cost to the environment.
There's obviously other issues like water consumption but using the above approach more rigorously might balance our diets a bit better by rearing only such livestock as can be fed on human-inedible vegetable matter.
ffm-
But those hillsides used to be wooded, and the felling of those trees to provide pasture contributes to soil erosion, reduction in biodiversity, and isn't terribly sustainable etc etc. There are problems however you look at it, but then that's why we're talking about a wholesale reduction in meat consumption.
TvH
I guess what you can say about being vegan, though, is that it's most compatible with a completely ethical and sustainable way of consuming food. Kind of like electric cars. In a way.