Very few moral philosophers have written on climate change.1 This is puzzling, for several reasons. First, many politicians and policy makers claim that climate change is not only the most serious environmental problem currently facing the world, but also one of the most important international problems per se.2 Second, many of those working in other disciplines describe climate change as fundamentally an ethical issue.3.

Commons problems are, understandably enough, typically thought to be problems about commons. In this paper, however, I argue that what generates some prominent examples of commons problems is not open access to a good. Instead, what generates some commons problems is a conflict of values that have different structures. After making this case, I show how the existence of such problems can motivate a version of the Precautionary Principle and a (qualified) rejection of cost-benefit analysis.