Paths to climate cooperation.

C limate change is the largest commons governance problem that humanity has ever faced. Emissions of greenhouse gases anywhere in the world contribute to radiative forcing everywhere. The impacts of climate change vary greatly from place to place, and the vulnerability to those impacts differs across human groups and across other species, even in a single location. The five nations with the largest CO2 emissions in 2008 were responsible for less than 60% of total global CO2 emissions (1). Although the action of a moderate number of the largest emitters could have some effect, substantial reductions in climate risk will depend on cooperative action across many nations. However, the costs of reductions are borne by each nation individually, whereas risk reduction is shared by all nations. Getting self-interested and often distrustful nations to cooperate is a major obstacle to addressing the climate problem. So far, international agreements have not had much impact on the trajectory of emissions or the concentration of greenhouse gases in the atmosphere. In the midst of this impasse, Heitzig et al. (2) offer an analysis that may show a way out, and they certainly suggest important avenues for additional analysis. Their work (2) falls clearly in what Stokes (3) labeled “Pasteur’s Quadrant”—like the work of Louis Pasteur, it contributes to fundamental knowledge and is clearly useful.