If sustainable development really can be more than an empty slogan, it will need to be powered by renewable energy sources like wind and solar. Renewables are growing quickly — by nearly 18% last year — but they still provide less than 3% of the world's energy supply. That's changing as nations like the U.S. and China place big bets on the renewable energy, even as ultra-green Germany leads the way with a plan to go 100% renewable by 2050. But wind and solar and other renewables really are going to displace fossil fuels — and eventually they must — we'll need technological leaps along with government support.