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How Do We Make Cities Sustainable?

From better mass transit to a stronger mix of renewable energy, what is the most important thing we can do to make cities smarter when it comes to energy use?

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Panama City glows at night

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 blight. Beijing's per-capita greenhouse gas emissions are higher than China's national average, and many U.S. cities are surrounded by suburbs with large carbon footprints. It will be important to pay attention to how we build cities, if we want metropolitan areas that make the planet a better place. (The Great Energy Challenge has addressed these issues at live events around the world. See a summary of the October 2014 in Tokyo, Japan, video perspectives from Tokyo; an earlier 2014 in Istanbul, "Powering Our Urban Future: Spotlight on Turkey," with video of the Istanbul participants discussing sustainable cities; and video highlights from experts at the February 2015 event in Jakarta, Indonesia.)

In your view, what is the most important thing we can do to make cities more livable and sustainable in their use of energy? Rate the ideas and comment below with your own.