Towards forward-looking carbon debt assessments to comprehensively capture state responsibility for climate change

Under current scenario projections of global climate ambition, we expect to exceed a 1.5°C consistent remaining carbon budget this decade. At the same time, many countries have already accrued ‘carbon debt’, reflecting historical emissions exceeding their ‘fair share’ of the remaining carbon budget. The ‘carbon debt’ concept can be extended to forward-looking scenario-based assessments, by comparing expected future emissions trajectories with current remaining budget allocations (or debts) for a given temperature target. At the year of net-zero carbon emissions, we term this the ‘net-zero carbon debt’. This measure can be used to assign responsibilities for expected temperature exceedance and related domestic climate impacts, capturing both intra- and inter-generational inequities. Such an approach ensures that ‘fair share’ considerations persist even if a remaining carbon budget is initially exhausted, linking expected emissions pathways with corresponding carbon drawdown obligations and responsibilities for realised impacts. We apply this approach to examine recent downscaled scenario projections for European Union member countries, quantifying the expected carbon drawdown obligations and responsibility for climate impacts that they imply. We then discuss the value and limitations of such assessments in informing domestic considerations of fairness thus far reliant on a rapidly dwindling remaining carbon budget. In summary, this work examines a new forward-looking domestic application of ‘fair share’ considerations in a manner robust to exhaustion of a 1.5°C consistent remaining carbon budget.