Forty years ago, a team of scientists at the Massachusetts Institute of Technology raised the first warnings about possible "limits to growth."

If human beings didn't slow the expansion of the size of our populations and economies, they predicted, sometime in the ensuing 100 years we would more or less fall off a cliff: Our environmental troubles would get so bad that our economies, and our civilization, would start to become unworkable.

For most of the four decades since, that analysis has drawn scorn. People insisted that human beings could keep our enterprise expanding forever — increases in knowledge would always trump a degrading physical world. We'd never run out of, say, oil — or if we did, we'd invent some substitute and just carry on as before.