The world's cities occupy 4 percent of the Earth's land area, yet they are home to more than half of the world's people. By 2030, that percentage will swell to 60 percent. Indeed, the United Nations projects that cities will absorb most of the world's population increase between now and 2050—more than two billion people—with the vast majority of urban expansion taking place in the developing world. (See related quiz: "What You Don't Know About Cities and Energy.")

Growing urbanization can be a plus for the environment, because people who live in dense cities drive less, their living spaces use less energy, and they require fewer resources. But there are also troubling trends, like increased traffic congestion, smog, and