"Climate change raises complex issues of science, economics, and politics; it also raises difficult issues of justice. Poor nations are especially vulnerable to rising temperatures, in part because they are poor. Wealthy nations have less at risk, in part because they are wealthy. It is both tempting and plausible to suggest that for either emissions reductions or adaptation, wealthy nations owe special obligations to poor ones. In this paper, we address this general question by focusing on a much narrower one: how should permits be allocated in a global cap-and-trade system?"