The quality of life of an animal comes before CO2, period! Especially when its billions of them to try to feed an ever growing population. People eat TOO much eat in many developed countries. There is no balance and its not even needed. In many countries, like India for example, many families eat meat once a week and then enjoy amazing vegetarian food the rest of the week..
Then also they REALLY enjoy that meat WAY more than you would if you ate it every day.. And guess what, 600% less animals had to die!