Philip Cafaro is a professor of philosophy at Colorado State University and co-editor of the anthology "Enough! Environmentalists Confront Population Growth," forthcoming from the University of Georgia Press.

Are declining birth rates a threat to the global economy? Only if you view “the economy” as an autonomous entity whose primary purpose is to grow as quickly as possible, and assume an infinite amount of ecological space in which it can do so. Both these assumptions are dubious.

Rather than bemoan the fact that reduced population growth might lead to less economic growth, we should try to lock in lower fertility levels.

Severe global environmental problems, including climate change, excessive nitrogen deposition in the oceans and mass species extinctions, suggest that the human economy is bumping up against ecological limits to growth. Even if we take increasing human economic activity as our primary goal, it isn’t clear how much longer pursuing it will be physically possible.

That’s all right, though, since the real purpose of an economy is not to grow ever larger, but to provide for people’s sustenance, security and well-being. We need to create economies that accomplish this without depending on growth.

If populations level off or decline, that removes one main incentive to pursue economic growth. Much of the remaining motivation might vanish, if we took seriously recent research showing that beyond a surprisingly modest level of affluence, increased wealth does not appear to make people happier or more secure.

In any case, when growing economies threaten ecological degradation that could seriously undermine people’s well-being, we seem to have reached a point where growth has become counterproductive.

Rather than bemoan the fact that reduced population growth might lead to less economic growth, we should try to lock in lower fertility levels, by providing inexpensive contraception and ensuring reproductive freedom as widely as possible. After all, while population growth rates are down, total human numbers continue to climb. Between 1999 and 2011, the global population grew by 1 billion people, with no end in sight.

Stabilizing, or better yet, reducing populations can help us create truly sustainable economies. And sustainability — not growth — is the key to human well-being in the 21st century and beyond.