ANDREA SALTELLI, PHILIP B. STARK, WILLIAM BECKER, PAWEL STANO Climate Models as Economic Guides: Scientific Challenge or Quixotic Quest? The uncertainties associated with mathematical models that assess the costs and benefits of climate change policy options are unknowable. Such models can be valuable guides to scientific inquiry, but they should not be used to guide climate policy decisions. In the polarized climate change debate, cost-benefit analyses of policy options are taking on an increasingly influential role. These analyses have been presented by authoritative scholars as a useful contribution to the debate. But models of climate—and especially models of the impact of climate policy—are theorists’ tools, not policy justification tools. The methods used to appraise model uncertainties give optimistic lower bounds on the true uncertainty, at best. Even in the finest modeling exercises, uncertainty in model structure is presented as known and manageable, when it is likely neither. Numbers arising from these modeling exercises should therefore not be presented as “facts” providing support to policy decisions. Building more complex models of climate will not necessarily reduce the uncertainties. Indeed, if previous experience is a guide, such models will reveal that current uncertainty estimates are unrealistically small. The fate of the evidence Climate change is the quintessential “wicked problem:” a knot in the uncomfortable area where uncertainty and disagreement about values affect the very framing of what the problem is. The issue of climate change has become so resonant and fraught that it speaks directly to our individual political and cultural identities. Scientists and other scholars often use non-scientific and value-laden rhetoric to emphasize to non-expert audiences what they believe to be the implications of their knowledge. For example, in Modelling the Climate System: An Overview, Gabriele Gramelsberger and Johann Feichter—after a sober discussion of statistical methods applicable to climate models—observe that “if mankind is unable to decide how to frame an appropriate response to climate change, nature will decide for both—environmental and economic calamities—as the economy is inextricably interconnected with the climate.” Historians Naomi Oreskes and Erik M. Conway, in their recent book The Collapse of Western Civilization (2014), paint an apocalyptic picture of the next 80 years, beginning with the “year of perpetual summer” in 2023, and mass-imprisonment of “alarmist” scientists in 2025. Estimates of the impact of climate change turn out to be far too cautious: global temperatures increase dramatically and the sea level rises by eight meters, resulting in plagues of devastating diseases and insects, mass-extinction, the overthrow of governments, and the annihilation of the human populations of Africa and Australia. In the aftermath, survivors take the names of climate scientists as their middle names in recognition of their heroic attempts to warn the world. That the Earth’s climate is changing, partly or largely because of anthropogenic emissions of CO 2 and other