Navigating the black box of fair national emissions targets

Abstract Current national emissions targets fall short of meeting the Paris Agreement climate goals, prompting increased attention on equitable ways to closing this gap in international climate negotiations. Fair emissions allocations are often determined using effort-sharing formulas based on principles of fairness, yielding diverse outcomes. This diversity in equity-based emissions allocations complicates efforts by policymakers and courts to discern the effects of assumptions and uncertainties from normative decisions. Here, we provide up-to-date fair emissions allocations in a comprehensive framework, accounting for considerations across three dimensions: physical and social uncertainties, global strategies and equity. The relative impact of these considerations is assessed using Sobol’s method, quantifying for which countries discussions on equity and socio-economic projections are important, and for whom global targets dominate. The fair emissions allocations are compared to NDCs and cost-optimal scenario output, identifying countries that have insufficient ambition and those for which – from a globally cost-optimal perspective – it is appealing to invest in mitigation abroad.