By 2050, 85 percent of people in the developed world and 63 percent of the developing world are expected to be urban dwellers. That creates either an opportunity to reduce emissions or exacerbate them, depending on the type of development we choose to undertake and how much people are prepared to share resources, including homes, transport, and durable goods. Sharing is the best bet cities have to fight climate change, but its going to require sharing on a much broader level than just Uber or Airbnb.