> Countries should be rewarded economically/politically by some sort of sustainability index where all of their positive/negative externalities are accounted for.

From which growth-based economies will those rewards be taken? If not that way, then how?

The whole point is a paradigm shift where sustainability is rewarded instead of uncontrolled growth. The reward can probably be political clout, to drive the direction of things moving forward. Essentially that is what we reward uncontrolled growth with currently.

The whole point is that I am assuming there isn't enough incentive on a large scale to deal with environment problem BEFORE they occur. With global warming, once we go past the point of no return, it won't be a quick fix. We will throw the equilibrium out of whack. If we were to pragmatically balance things now/going back a few decades, we could avoid the big swing that will occur.

So basically, the population may fall from 8 billion to 3 billion because of famine/war, etc. Immensely painful things... Or we could just have pragmatic restraint through rewarding sustainability, and it would naturally level out to 5/6 billion or whatever that number is... but without the immense pain of billion fighting/dying.