COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC. known as the Copenhagen Consensus ranked the world’s most pressing environmental, health and social problems in a prioritized list. Assembled by the Danish Environmental Assessment Institute under its then director, Bjørn Lomborg, the panel used cost-benefit analysis to evaluate where a limited amount of money would do the most good. It concluded that the highest priority should go to immediate concerns with relatively well understood cures, such as control of malaria. Long-term challenges such as climate change, where the path forward and even the scope of the threat remain unclear, ranked lower. Usually each of these problems is treated in isolation, as though humanity had the luxury of dealing with its problems one by one. The Copenhagen Consensus used stateof-the-art techniques to try to bring a broader perspective. In so doing, however, it revealed how the state of the art fails to grapple with a simple fact: the future is uncertain. Attempts to predict it have a checkered history—from declarations that humans would never fly, to the doom-and-gloom economic and environmental forecasts of the 1970s, to claims that the “New Economy” would do away with economic ups and downs. Not surprisingly, those who make decisions tend to stay focused on the next fiscal quarter, the next year, the next election. Feeling unsure of their compass, they hug the familiar shore. This understandable response to an uncertain future means, however, that the nation’s and the world’s long-term threats often get ignored altogether or are even made worse by shortsighted decisions. In everyday life, responsible people look out for the long term despite the needs of the here and now: we do homework, we save for retirement, we take out insurance. The same principles should surely apply to society as a whole. But how can leaders weigh the present against the future? How can they avoid being paralyzed by scientific uncertainty? In well-understood situations, science can reliably predict the implications of alternative policy choices. These predictions, combined with formal methods of decision analysis that use mathematical models and statistical methods to determine optimal courses of action, can specify the trade-offs that society must inevitably make. Corporate executives and elected officials may not always heed this advice, but they do so more often than a cynic might suppose. Analysis has done much to improve the quality of lawmaking, regulation and investment. National economic policy is one example. Concepts introduced by analysts in the 1930s and 1940s—unemployment rate, current-account deficit and gross national