they take a myopic view of the inputs and outputs for food sources, not considering, for instance, that much of what is fed to animals would otherwise be wasted. the beef doesn't produce all that CO2, poore & nemecek were calculating all the co2 that goes into the inputs. i mentioned elsewhere cottonseed, but frankly i know that only takes up a minute portion of what they're calculating. instead, they are also counting soy, and that's almost as dishonest as you can get. nearly all soy is pressed for oil, and after that, the waste product is what is fed to cattle and other livestock. technically, you could eat it, but most people don't and don't want to. feeding it to livestock actually reclaims waste products. and even the calculation for the soy itself is skewed since it often also counts the deforestation that has already taken place as an emission source, regardless of whether that particular plot of land has been deforested for decades.