Something I tell libertarians that a few of us lefties could stand to hear...

[Permanut]

2. We have evidence..

that the carrying capacity of the Earth is being exceeded:

http://en.wikipedia.org/wiki/Carrying_capacity

Carrying capacity includes food, habitat, water, and other necessities. If we have found a way to monkey with the food supply, we have not found an elegant way to provide water for all, and rather than enhancing our habitat, we are destroying it.