The planet sure seems smaller and smaller these days. The "wide-open spaces" that the Grammy-winning Dixie Chicks sing about are becoming few and far between. In little more than a century, humanity has gone from the agrarian age to the age of megacities. Four decades ago, there were only three cities with more than 8 million people: New York, London and Tokyo. By 2015 there will be 33 such cities, 27 of them — like Cairo — in the developing world. The urbanization of the globe is more than an aesthetic problem. Human sprawl threatens the habitat of most animal and plant species — except for cockroaches, rats, pigeons, crabgrass and other organisms that thrive with mankind. Relentless human expansion is the main reason the world is fast losing its biodiversity, raising the specter that we will eventually live, in the words of writer David Quammen, on a "planet of weeds." If that danger doesn't seem imminent, consider this: sprawl is paving over the land we need to grow our food. Since 1981 the amount of land around the world devoted to raising grain has fallen 7%. Increased agricultural productivity has made up for that loss, but the Green Revolution may be reaching the point of diminishing returns. In 1998 the world grain harvest declined 2% from the previous year, even as there were 1.4% more mouths to feed.

Sprawl is understandable, maybe even unavoidable, in countries where the population is still growing rapidly. But it is more difficult to explain in the U.S. and other rich countries with lower birthrates. In Ohio the amount of land developed around urban areas between 1960 and 1990 grew more than five times as fast as the population.

Maybe it's just a response to endless complaints about suburban traffic jams, but U.S. politicians are starting to pay attention to the sprawl problem. Presidential candidate Al Gore has raised the subject, and Maryland Governor Parris Glendening sounds downright alarmed. "Every time we cut down one more forest or sell off another acre of farmland, we have permanently lost more of our finite natural resources," says Glendening. "Sprawl costs taxpayers dollars to support new infrastructure, costs natural resources that we know are not unlimited, and costs us as a society in lost opportunities to invest in our existing communities and neighborhoods." No one has an easy way to eliminate sprawl, but there are at least four strategies for containing it: