The global economy isn't a zero-sum game, like I don't need to become more poor in order for someone else to become more rich. But growth, at least so far, has been dependent upon unsustainable use of the planet's resources. The planet can't sustain 7 billion automobiles, for instance, or 7 billion frequent flyers, although most of us who can afford to drive or fly feel entitled to do so. You'll remember that when we talked about the Industrial Revolution, we discussed the virtuous cycle of more efficiency making things cheaper, which in turn made them easier to buy, which increased demand, which increased efficiency. But from the perspective of the planet, each turn in that cycle takes something. More land under cultivation, more carbon emissions, more resource extraction. That can't go on forever, but worryingly, our current models of economic growth don't allow for any other way.