We can think of our planet’s vast store of resources and ecosystem services—Earth’s natural capital—as a bank account. To keep a bank account full, we need to leave the principal intact and spend just the interest, so that we can continue living off the account far into the future. If we begin depleting the principal, we draw down the bank account.

To live off nature’s interest—the renewable resources that are replenished year after year—is sustainable. To draw down resources faster than they are replaced is to eat into nature’s capital, the bank account for our planet and our civilization. Currently we are drawing down Earth’s natural capital—and we cannot get away with this for long. Historical evidence suggests that civilizations can crumble when pressures from population and consumption overwhelm resource availability.

Historians have inferred that environmental degradation contributed to the fall of the Greek and Roman empires; the Angkor civilization of Southeast Asia; and the Maya, Anasazi, and other civilizations of the Americas. In Syria, Iraq, and elsewhere in the Middle East, areas that today are barren desert had earlier been lush enough to support the origin of agriculture and thriving ancient societies. Easter Island has long been held up as a society that self-destructed after depleting its resources, although new research paints a more complex picture.