World3—the computer program used in the Limits to Growth study, commissioned by the Club of Rome and published in 1972 by a team of MIT researchers under the direction of the famed Jay Forrester—was the first serious effort to model humanity and its natural environment as a single, integrated, global system. In World3, worsening scarcity of nonrenewable resources was the main driver of global crisis. However, the distinction between nonrenewable and renewable resources is misleading, in part because the very label “renewable” suggests that such resources are more abundant and less vulnerable to degradation than nonrenewables. Dr Homer-Dixon will show that the critical distinction is between what we can call “complex” and “noncomplex” resources. Complex resources—including Earth’s climate and the planet’s cycles of fresh water, nitrogen, and carbon, as well as its living systems, like fisheries, forests, and pollinators—have key features that make them particularly vulnerable to over exploitation and degradation.