The problem of climate change seems to be a tragedy of the commons: despite the global benefits of reducing green-house gas emissions, no individual has any incentive to reduce his or her own emissions. Yet many people are making efforts to reduce emissions and putting pressure on businesses and governments to do the same. Although the size of these efforts is unclear, their very existence might seem puzzling. The efforts are consistent, however, with some theoretical and empirical evidence about the extent of cooperation in other social dilemmas. This evidence does not imply that greenhouse-gas emissions will be reduced to desirable levels, but it does suggest that the potential for voluntary cooperation should not be ignored. It also suggests that cooperation can be promoted by (i) allowing cooperators to punish defectors without withdrawing their own cooperation
(ii) publicly emphasizing the social benefits and extent of cooperation and the social norms that require it
and (iii) improving the quantity and timeliness of public information about cooperation and defection.