The planet's biggest problems have to do with sustainability, environmental decline, global poverty, disease, conflict and so forth. Really, they're all interconnected - it's one big problem, which is that the way we're doing things can't go on.

Cities offer us powerful leverage on our most stubborn, wasteful practices. Long commutes in our cars, big power bills from our energy-hogging buildings, shopping trips to buy stuff that'll spend a few short months in our homes and long centuries in our landfills.

There are a lot of different ways of building a prosperous society, and some of them use much less energy than others. And it is possible and more practical to talk about rebuilding systems to use much less energy than it is to think about trying to meet greater demands of energy through clean energy alone.

The biggest thing growing cities need to do is minimize barriers to development so that as long as someone is doing good urbanism, they can get permitted quickly and get building quickly.