Cities are home to more than half the world's population, and they exert increasing stress on the earth. They produce up to 70 percent of global carbon dioxide emissions, use up vast quantities of water, degrade water quality and produce mountains of waste. As cities go, so goes the planet. And cities are growing—fast. By 2030, according to the latest United Nations estimates, five billion people will live in cities, nearly half of them conducting their lives in homes, schools, workplaces and parks that do not yet exist.