Delayed carbon debt drawdown fails younger generations

Current emissions trends may deplete a 1.5°C consistent carbon budget in approximately 6 years, resulting in a period of exceedance. To inform the equitable realignment of efforts that minimize exceedance magnitude and duration, we propose the use of ‘net-zero carbon debt’. Net-zero carbon debt is a measure that combines ‘fair’ shares of a remaining carbon budget with carbon drawdown responsibilities during the period of exceedance. We apply this measure to evaluate a set of regional emissions scenarios ranging from current policies to targets and pledges, considering the per capita allocation of a remaining 1.5°C carbon budget from the year 1990. Quantifications of regional net-zero carbon debt are used to attribute regional responsibility for exceedance drawdown and proportional responsibility for regional climate impacts if this does not occur. Even under our most optimistic assessment of current global ambition, younger generations in regions accruing net-zero carbon debts inherit annual average exceedance drawdown responsibilities ranging from 0.7-5.6 GtCO2/year between 2050-2100, while those born in 2020 in these regions are expected to experience 1.1-1.9 times increase in lifetime extreme heatwave exposure relative to a reference 1.5°C scenario if this drawdown does not occur. This analysis can inform the operationalization of equitable intertemporal and interregional effort sharing mechanisms that persist beyond exhaustion of a remaining carbon budget.