Non-CO₂ forcing changes will likely decrease the remaining carbon budget for 1.5 °C

One key contribution to the wide range of 1.5 °C carbon budgets among recent studies is the non-CO₂ climate forcing scenario uncertainty. Based on a partitioning of historical non-CO₂ forcing, we show that currently there is a net negative non-CO₂ forcing from fossil fuel combustion (FFC), and a net positive non-CO₂ climate forcing from land-use change (LUC) and agricultural activities. We perform a set of future simulations in which we prescribed a 1.5 °C temperature stabilisation trajectory, and diagnosed the resulting 1.5 °C carbon budgets. Using the historical partitioning, we then prescribed adjusted non-CO₂ forcing scenarios consistent with our model's simulated decrease in FFC CO₂ emissions. We compared the diagnosed carbon budgets from these adjusted scenarios to those resulting from the default RCP scenario's non-CO₂ forcing, and to a scenario in which proportionality between future CO₂ and non-CO₂ forcing is assumed. We find a wide range of carbon budget estimates across scenarios, with the largest budget emerging from the scenario with assumed proportionality of CO₂ and non-CO₂ forcing. Furthermore, our adjusted-RCP scenarios produce carbon budgets that are smaller than the corresponding default RCP scenarios. Our results suggest that ambitious mitigation scenarios will likely be characterised by an increasing contribution of non-CO₂ forcing, and that an assumption of continued proportionality between CO₂ and non-CO₂ forcing would lead to an overestimate of the remaining carbon budget. Maintaining such proportionality under ambitious fossil fuel mitigation would require mitigation of non-CO₂ emissions at a rate that is substantially faster than found in the standard RCP scenarios.