Using arguments that parallel those of Blaise Pascal and William James, Haller offers prudential reasons for caution that should convince those not already persuaded by ethical arguments. While models of global systems can reveal only possible, not probable, futures, the catastrophic threats posed by such things as global warming, ozone depletion, or population increase represent what James would call "live options": that is, they present us with a plausible possibility that forces us to make momentous decisions. Haller concludes that we cannot afford to risk catastrophe, despite the high costs this decision involves.