Remember how Wile E. Coyote, in his obsessive pursuit of the Road Runner, would fall off a cliff? The hapless predator ran straight out off the edge, stopped in midair as only an animated character could, looked beneath him in an eye-popping moment of truth, and plummeted straight down into a puff of dust. Splat! Four decades ago, a Massachusetts Institute of Technology computer model called World3 warned of such a possible course for human civilization in the 21st century. In Limits to Growth , a bitterly disputed 1972 book that explicated these findings, researchers argued that the global industrial system has so much inertia that it cannot readily correct course in response to signals of planetary stress. But unless economic growth skidded to a halt before reaching the edge, they warned, society was headed for overshoot–and a splat that could kill billions.