Think of the problem of climate change as a game between two countries called China and the US, considered as if each were a single individual. Each country has two possible strategies for addressing global carbon emissions:

1. Restrict: taking measures to reduce emissions, for example by taxing the use of fossil fuels
2. BAU (business as usual): a scenario in which people, governments and businesses are free to pursue their own pleasures, politics, and profits without taking adequate account of the effect of their actions on others, including future generations.

The figures below describe the outcomes and hypothetical payoffs of this game.

US US Restrict BAU Restrict BAU Reduction in BEST GOOD Restrict emissions sufficient US free rides on to moderate Chinese emissions climate change cutbacks Restrict BEST WORST China China China free rides on WORST BAD BAU US emissions No reduction cutbacks in emissions BAU GOOD BAD