Cannibalism, lol. As soon as we finish unlocking fusion we will have more than enough clean power to feed everyone using aquaponics.
I always felt this quote appropriate.
Agent Smith: I'd like to share a revelation that I've had during my time here. It came to me when I tried to classify your species and I realized that you're not actually mammals. Every mammal on this planet instinctively develops a natural equilibrium with the surrounding environment but you humans do not. You move to an area and you multiply and multiply until every natural resource is consumed and the only way you can survive is to spread to another area. There is another organism on this planet that follows the same pattern. Do you know what it is? A virus. Human beings are a disease, a cancer of this planet. You're a plague and we are the cure.
I am not a tree hugger or anything its just that what ever it is we do, we over do it. I always felt that there has to be a balance between science and spirit, to far to either side and you fail.