Animal agriculture animals eat a lot of the parts of food that humans cannot or will not eat, so it’s efficient. Much of the roughage they eat is grown on land that is not useable for anything else. Plants use a lot of water as well and I would say animal products give out a lot more energy per unit than most plants. That is why I think looking at intensities is a better than looking at pure output. Fossil carbons would stay in the ground forever if it weren’t for humans digging it up. Plants are made from carbon already in the air from animal respiration and digestion. So what goes into the air will be the same carbon atoms that will eventually go back to their source, plants. Unlike fossil carbons, which add to the atmosphere carbon atoms that cannot go back to their source underground.