“Let me add to the complexity of the situation we find ourselves in: At the same time we’re solving for climate change, we’re going to be building cities for three billion people,” said the urban designer Peter Calthorpe at the beginning of his talk at TED 2017. By 2050, he said, we will see the urban population double, and if we don’t manage to sustainably and practically build to accommodate that growth, “I’m not sure that all the climate solutions in the world will help save mankind,” Calthorpe said. But there is a real enemy to target in this situation, Calthorpe said, and it is sprawl.