Towards a Methodology for Specifying States' Legal Obligations to Reduce Greenhouse Gas Emissions in Line with Equity and Best Available Science

Specifying states' obligations to reduce greenhouse gas emissions in line with equity and best available science requires addressing the following interrelated questions: Is there a legally operationalizable way of equitably distributing a global carbon budget among states, and if so, which global carbon budget must states not exceed? Political discretion in answering these questions commonly relies on two premises: Relevant scientific findings come with considerable uncertainty and choosing emission (reduction) levels is a normative undertaking belonging to the political domain. This thesis explores this reasoning by engaging with the state of relevant knowledge in the fields of atmospheric science, statistics and climate economics and translating the insights gained to the legal discipline. The aim is to narrow extant uncertainties about how to specify states' obligations to reduce emissions in line with best available science and equity, thereby also strengthening the judicial reviewability of emission policies.

To this end, the thesis proceeds by means of a three-tier approach. Paper I focuses on the question of how to equitably distribute a global carbon budget and examines relevant statistical tools as to their legal operationalizability. The paper finds that the so-called PRIMAP Equity tool - used by the Climate Action Tracker and developed by two of its founding institutions - is the best available such tool and apt for legal use.

The remainder of the thesis turns to the question of what maximum global temperature rise (and corresponding global carbon budget) states are to align their policies with in the first place. In this vein, Paper II engages with leading studies in the field of atmospheric science and examines the extant state of knowledge and uncertainty about the relationship between global warming levels and climate tipping points from a due diligence and equity perspective. Paper III builds on this analysis by exploring economic cost-benefit models aimed at calculating the 'optimal' level of global warming from a proportionality perspective. In light of the severity and foreseeability of harm and states' cumulative capacity to close the emissions gap, the thesis concludes that it would be proportionate in the strict sense for states to cumulatively align their climate policies with a maximum 1.5°C temperature rise.