Using net-zero carbon debt to track climate overshoot responsibility

Abstract Current emissions trends are likely to deplete a 1.5°C consistent carbon budget soon after the year 2030, resulting in a period of overshoot. To navigate responsibilities for and during this period we contrast ‘fair’ allocations of a remaining carbon budget with projected carbon emissions trends until net-zero. We term this measure the ‘net-zero carbon debt’, or the expected overconsumption of a ‘fair’ allocation in the year a party achieves net-zero carbon emissions. The ‘net-zero carbon debt’ measure thus provides an estimate today of a party’s expected responsibility for climate overshoot under certain considerations of fairness. We apply this measure to assess all vetted mitigation scenarios evaluated in the IPCC’s sixth assessment report and two bespoke scenarios that assess current policies and net-zero targets, considering the equal per capita allocation of a remaining 1.5°C carbon budget from the year 1990. The resulting regional net-zero carbon debt quantifications are used to determine regional: i) responsibility for overshoot, ii) exceedance drawdown obligations, and iii) increase in extreme climate exposure if drawdown does not occur. Our work generates a new domestic signal from a typically international climate equity discourse and can inform the operationalization of intergenerational and interregional effort sharing mechanisms that persist beyond exhaustion of a remaining carbon budget.