Historical Emissions and the Carbon Budget

How should the world's remaining carbon budget be divided among countries? We assess the role of a fault-based principle in answering this question. Discussion of the role of historical emissions in dividing the global carbon budget has tended to focus on emissions before 1990. We think that this is in part because 1990 seems so recent, and thus post-1990 emissions seem to constitute a lesser portion of historical emissions. This point of view was undoubtedly warranted in the early 1990s, when discussion of fault-based principles in this context began. While this view still has some intuitive force, we find that it and the associated focus on pre-1990 emissions are now out of date. Emissions since 1990 in fact constitute a large and rapidly increasing proportion of emissions since 1750 - approximately half of the carbon emissions due to fossil fuel use and cement production, at the time of writing. We show that a restricted fault-based principle, according to which emissions should be divided among countries on the basis of their emissions since 1990, is both viable and powerful. We consider standard objections to a fault-based principle in this context, how such a principle might more concretely be applied, and its likely implications.