Compelling research in international relations and international political economy on global warming suggests that one part of any meaningful effort to radically reverse current trends of increasing green house gas (GHG) emissions is shared policies among states that generate costs for such emissions in many if not most of the world’s regions. Effectively employing such policies involves gaining much more extensive global commitments and developing much stronger compliance mechanism than those currently found in the Kyoto Protocol. In other words, global warming raises the prospect that we need a global form of political authority that could coordinate the actions of states in order to address this environmental threat. This in turn suggests that any serious effort to mitigate climate change will entail new limits on the sovereignty of states. In this book I focus on the normative question of whether or not we have clear moral reasons to bind ourselves together in such a supranational form of political association. I argue that one can employ familiar liberal arguments for the moral legitimacy of political order at the state level to show that we do have a duty to support such a global political project. Even if one adopts the premises employed by the most influential forms of liberal scepticism to the ideas of global political and distributive justice, such as those advanced by John Rawls and Thomas Nagel, it is clear that the threat of global warming has expanded the scope of justice. We now have a global and demanding duty of justice to create the political conditions that would allow us to collectively address our impact on the Earth’s atmosphere.