Cities are an integral part of Earth’s future: by 2050, 68 percent of the world’s population will be living in an urban area. Solutions to social problems, from climate change to poverty, will therefore be tied to the fates of cities. In this episode, Glimp professor of economics Ed Glaeser explains why he is overwhelmingly optimistic about urban growth. Cities, he says, are engines of innovation and economic activity that create opportunity. “Humans are a social species that gets smart,” he explains, “by being around other smart people.” When they do, their impact on the planet’s climate is lessened in surprising ways—and in surprising places across the United States.