> If you wanted to replace all the meat and dairy with equivalent protein substitutes you’d be looking at huge amounts of land for growing soybeans, peanuts, tree nuts

In addition to the other replies, I'd like to point out that insect farming is an often overlooked solution. But of course it's just not sexy enough for most westerners.

According to the Food and Agriculture Organization of the United Nations:

> Edible insects contain high quality protein, vitamins and amino acids for humans. Insects have a high food conversion rate, e.g. crickets need six times less feed than cattle, four times less than sheep, and twice less than pigs and broiler chickens to produce the same amount of protein. Besides, they emit less greenhouse gases and ammonia than conventional livestock. Insects can be grown on organic waste. Therefore, insects are a potential source for conventional production (mini-livestock) of protein, either for direct human consumption, or indirectly in recomposed foods (with extracted protein from insects); and as a protein source into feedstock mixtures.

http://www.fao.org/edible-insects/en/