The problem is that the global population base has increased so radically that even seemingly modest birthrates can have momentous consequences. Joel Cohen (head of the Laboratory of Populations at the Rockefeller University and Columbia University) calculates that if we do add 2.5 billion people by 2050, and virtually all the increase, as expected, goes into poor countries’ cities, then the world will have to build one city of one million people every week for the next 43 years. "Is this," he asks, "feasible — physically, environmentally, financially, socially?"

One sort of shudders at the answer. But there is a first step: get a handle on growth of the world’s cities. Without that, how can city leaders estimate the peripheral areas they’ll have to urbanize, or, alternatively how much they’ll have to "infill" their current territory with higher density development?

The bottom line is clear: the developing world’s cities — and the developed world’s cities still expanding significantly — must plan early, much more carefully, or expect to be overwhelmed by a virtual growth tsunami.

Good planning, for example, can recycle underused urban land, or schedule better use of expansion areas, to achieve much greater people-carrying capacity. Good planning can avoid some of the worst modern traffic jams, put public transit first, make walking and biking convenient, and preserve pockets of "green" critical to humans’ physical and emotional health.

Photo: Mexico City, by dantebusquets/Flickr