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

Estimates of the 1.5°C carbon budget vary widely among recent studies. One key contribution to this range is the non-CO₂ climate forcing scenario uncertainty. Based on a partitioning of historical non-CO₂ forcing, we show that there is currently a net negative non-CO₂ forcing from fossil fuel combustion (FFC) mainly due to the co-emission of aerosols, and a net positive non-CO₂ climate forcing from land-use change (LUC) and agricultural activities. We then perform a set of future simulations in which we prescribed a 1.5°C temperature stabilization trajectory, and diagnosed the resulting 1.5°C carbon budgets. Using the results of our historical partitioning, we prescribed changing non-CO₂ forcing scenarios that are consistent with our model’s simulated decrease in FFC CO₂ emissions. We compared the diagnosed carbon budgets from these idealized scenarios to those resulting from the default RCP scenario non-CO₂ forcing, as well as from a scenario in which we assumed proportionality between future CO₂ and non-CO₂ forcing. We find a large 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, in which the non-CO₂ forcing is consistent with model-diagnosed FFC CO₂ emissions, produced carbon budgets that are smaller than the corresponding default RCP scenarios. Our results suggest that ambitious mitigation scenarios will likely be characterized 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 required to avoid low-temperature targets. Maintaining such proportionality under ambitious fossil fuel mitigation would require mitigation of non-CO₂ emissions from agriculture and other non-FFC sources at a rate that is substantially faster than is found in the standard RCP scenarios.