History will have to judge whether the world is now following the path of Mauritius or Babylon. At least there are signs of action. In the 1970s a new strain of populist environmental activist began to usurp the clubby gentlemen who had previously led the movement, changing priorities from protecting wildlands to a more health-oriented agenda of clean air and water. Litigious American groups like the Environmental Defense Fund made certain that the laws were enforced, while Green parties sprang up throughout Europe. Today the Hudson River is relatively clean, and New York harbor has been so rejuvenated that wood-boring worms, long checked by pollution, thrive again in Gotham's remaining wooden piers.

In keeping with historical tradition, the poorer nations repeated the mistakes of their industrialized neighbors even as the rich countries were changing their ways. From Mexico to China, megacities in emerging nations recreated the Stygian pall of air pollution and the stinking waterways that the industrial world was busily trying to clean up, offering the excuse that a clean environment was a luxury for the rich.

Now there is reason to believe that both rich and poor nations are trying to break the ancient custom of pollute first, pay later. For one thing, the stakes have changed. At some point in this century, humanity itself became a geophysical force, able to affect the fundamental systems that run the planet. Educated people have long known that nuclear weapons have the potential to wipe out most life on land. In recent decades it has become clear that humanity could accomplish this same horrific feat inadvertently in the ordinary course of economic development.