Title: Climate uncertainty impacts on optimal mitigation pathways and social cost of carbon Show Chinese title

Title: 气候不确定性对最优减缓路径和社会碳成本的影响

Abstract: Emissions pathways used in climate policy analysis are often derived from integrated assessment models. However, such emissions pathways do not typically include climate feedbacks on socioeconomic systems and by extension do not consider climate uncertainty in their construction. We use a well-known cost-benefit integrated assessment model, the Dynamic Integrated Climate-Economy (DICE) model, with its climate component replaced by the Finite-amplitude Impulse Response (FaIR) model (v2.1). The climate uncertainty in FaIR is sampled with an ensemble that is consistent with historically observed climate and Intergovernmental Panel on Climate Change (IPCC) assessed ranges of key climate variables such as equilibrium climate sensitivity. Three scenarios are produced: a pathway similar to the "optimal welfare" scenario of DICE that has similar warming outcomes to current policies, and pathways that limit warming to "well-below" 2C and 1.5C with low overshoot, in line with Paris Agreement long-term temperature goals. Climate uncertainty alone is responsible for a factor of five variation (5-95% range) in the social cost of carbon in the 1.5C scenario. CO2 emissions trajectories resulting from the optimal level of emissions abatement in all pathways are also sensitive to climate uncertainty, with 2050 emissions ranging from -12 to +14 GtCO2/yr in the 1.5C scenario. Equilibrium climate sensitivity and the strength of present-day aerosol effective radiative forcing are strong determinants of social cost of carbon and mid-century CO2 emissions. This shows that narrowing climate uncertainty leads to more refined estimates for the social cost of carbon and provides more certainty about the optimal rate of emissions abatement. Including climate and climate uncertainty in integrated assessment model derived emissions scenarios would address a key missing feedback in scenario construction. Show Chinese abstract

Abstract: 在气候政策分析中使用的排放路径通常来自综合评估模型。 然而，这样的排放路径通常不包括对社会经济系统的气候反馈，因此在构建时也不考虑气候不确定性。 我们使用了一个著名的成本-收益综合评估模型，即动态综合气候-经济（DICE）模型，并将其气候部分替换为有限振幅脉冲响应（FaIR）模型（v2.1）。 FaIR中的气候不确定性通过一个与历史观测的气候和政府间气候变化专门委员会（IPCC）评估的关键气候变量范围（如平衡气候敏感性）一致的集合进行采样。 生成了三个情景：一个类似于DICE的“最优福利”情景，其升温结果与当前政策相似，以及旨在将升温限制在“远低于”2°C和1.5°C且低超调的情景，符合巴黎协定的长期温度目标。 仅气候不确定性就导致了1.5°C情景中碳的社会成本五倍的变化（5-95%范围）。 所有路径中最优减排水平所产生的CO2排放轨迹对气候不确定性也十分敏感，在1.5°C情景中，2050年的排放量范围从-12到+14 GtCO2/yr。 平衡气候敏感性和当前气溶胶有效辐射强迫的强度是碳的社会成本和本世纪中叶CO2排放的重要决定因素。 这表明，缩小气候不确定性可以更精确地估算碳的社会成本，并提高对最优减排率的确定性。 在综合评估模型生成的排放情景中包含气候和气候不确定性，将解决情景构建中的一个关键缺失反馈。