How might climate change affect the acidification of the world’s oceans or air quality in China and India in the coming decades, and what climate policies could be effective in minimizing such impacts? To answer such questions, decision makers routinely rely on science-based projections of physical and economic impacts of climate change on selected regions and economic sectors. But the projections they obtain may not be as reliable or useful as they appear: Today’s gold standard for climate impact assessments—model intercomparison projects (MIPs)—fall short in many ways.