I grew up in a world of rural nations where forests and farmland seemed infinite. Now most are urban nations. Indeed, at this moment the globe zooms over a threshold; half of us (three billion people) now aggregate in and around cities.

Yet the big change lies just ahead. In a single generation two billion more people are expected to join the urban population. Where will they live their lives? In much bigger and more numerous urban regions? Next door? Like an urban tsunami, easily visible today, we sweep swiftly and powerfully across our finite land.

For years I asked people in audiences to visualize the place where they grew up, and indicate whether it is better or worse now. Minds instantly left the room, speeding through images of memorable neighborhoods, glorious experiences, tough times, meaningful spots, and inspiring nature. Upon rapid return, virtually all audiences on different continents agreed: 80 to 90% of their formative landscapes are worse today. Yet this trend could be turned around. Incremental solutions crowd our plate, while promising big-picture solutions increasingly appear, often ready for serious evaluation or action.

One of the great challenges of history has appeared, the giant urban region. At the center, a huge city population depends fundamentally and daily on resources that are out of sight, out of the city. An engineering and architectural marvel, the city expands at its edges or along transportation corridors or dispersed as sprawl.