Despite decades of copious research on the subject, environmental scientists are still flummoxed by their own uncertainty—even ignorance—when it comes answering central questions about the future of the climate. What will happen if no global agreement emerges to limit greenhouse gas emissions? And what will be the likely impact of climate change on economies and societies? Wagner and Weitzman focus on several low-probability but high-cost outcomes, such as a rise in average surface temperature of six degrees, rather than the 2.5- or 3-degree increase that more conservative estimates foresee. They argue that governments should do what they can now to avoid the most devastating outcomes, even if they are less likely, and suggest as a model the kind of worst-case planning that sometimes informs national security policies and budgets. The book combines sophisticated analysis with a breezy, informal style, relying (not always convincingly) on analogies rather than equations to drive home its points.