The argument seems to be that livestock only eats stuff that would have been grown anyway and therefore has no effect on CO2 emissions. But I think this is wron

The argument seems to be that livestock only eats stuff that would have been grown anyway and therefore has no effect on CO2 emissions. But I think this is wrong for two reasons: 1. The premise is false. Quite a lot of farmland is devoted to growing feed grains. See https://www.ers.usda.gov/topics/crops/corn-and-other-feed-grains/feed-grains-sector-at-a-glance/ https://www.ers.usda.gov/topics/crops/corn-and-other-feed-gr... If we had less livestock, we could either grow human-edible crops instead (with much less energy per calorie expended) or leave these fields fallow. 2. It ignores the fact that some livestock (cows) converts feed into methane, which has a disproportionate climate impact—more methane than if the same grain had been produced, but not consumed by a cow. Admittedly, this is less relevant when we're talking about lab-grown chicken meat. But GP's point was about livestock generally, so this is worth considering.