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.

在最近的研究中，广泛的1.5°C碳预算的一项主要贡献是非CO ₂气候强迫情景的不确定性。基于对历史非CO ₂强迫的划分，我们显示当前存在来自化石燃料燃烧（FFC）的净负非CO ₂强迫，以及来自土地利用变化的净正非CO ₂气候强迫（ LUC）和农业活动。我们执行了一系列未来的模拟，其中我们规定了1.5°C的温度稳定轨迹，并诊断了由此产生的1.5°C的碳预算。然后，使用历史分区，我们规定了调整后的非CO ₂强迫情景，该情景与我们模型对FFC CO ₂的模拟减少相一致排放。我们将这些调整后情景中的诊断碳预算与默认RCP情景中非CO ₂强迫产生的碳预算进行了比较，并与假设未来CO ₂和非CO ₂强迫之间存在比例关系的情景进行了比较。我们发现各种方案之间的碳预算估算范围很广，其中最大的预算来自假定CO ₂和非CO ₂强迫成比例的方案。此外，我们的调整后RCP方案产生的碳预算小于相应的默认RCP方案。我们的结果表明，雄心勃勃的缓解方案可能会以非CO ₂贡献增加为特征。假设CO ₂与非CO ₂强迫之间持续成比例，将导致剩余碳预算的高估。要在雄心勃勃的化石燃料减排中维持如此的比例，就需要以比标准RCP情景中发现的速度快得多的速率来缓解非CO ₂排放。