Environmental threats like carbon emissions do not respect political boundaries. A ton of carbon emitted in Santiago affects climate change the same way as a ton of carbon emitted in Cape Town. Given the global impacts of international public goods, countries should work together: the responsibility to reduce risks to global public goods must ultimately be shared among countries that benefit from those public goods? The challenge raises the classic dilemma: although nations have a common interest to protect themselves, they also have a private incentive to abate pollution as they see fit, which might not match what is best for the globe. For example, the benefits generated by carbon abatement in Japan cannot be denied to anyone anywhere: it is a global public good covering all countries.