The resources spent to grow and maintain the animals we consume are unsustainable, the grounds become infertile, huge ammounts of water is used, and much more. Also a lot of polution comes from the food industry. All those animals release a gigantic ammount of methane (which is not all). CO2, although its the most mentioned greenhouse gas in excess right round, is obviously not the only culprit in that department. The least harmful food industries are aviaries, white meats. it is possible to take all the necessary nutrients from vegetals and such, although harder of course. Meat could stay in business if it's consumption was drastically reduced. Also synthetic foods have already been developed, and improvements are being made regarding its taste. That should replace real food. But since they don't teach useful stuff to politicians and they cannot/don't want to comprehend basic high school education, things are likely to remain the same. Also there is of course a lot of money in the food industry. Maybe their children can spend that money on Mars. Feel free to disagree.