Why the social cost of carbon increases when governments fail:The importance of inequality and institutional constraints for the social cost of carbon as a normative guide for climate policy

碳的社会成本(SCC)一直是气候政策的核心指标。SCC衡量的是当碳排放减少一个单位时避免的货币气候损害。SCC的水平在很大程度上取决于关于正义的规范性假设：如果贫困家庭的气候损害不能通过富裕家庭的再分配得到补偿，那么全球对公平的偏好可能会显著增加SCC。然而，大多数估计SCC的研究都假设有有效的再分配工具来平衡家庭收入不平等和气候损害。未来，SCC可能会对监管分析变得越来越重要，以从经济上证明雄心勃勃的气候政策措施的合理性。为了增加SCC估计的稳健性，它们的推导需要考虑到现实世界对分布的限制和经验水平的不平等。该项目开发了一种用于计算SCC的新的两级治理模型，允许将社会对公平的偏好与家庭一级的不平等结合起来。在全球治理层面，SCC被确定为最大化社会福利功能的最优碳税。国家治理水平在家庭之间重新分配。国家转移导致一定程度的家庭不平等，在确定SCC时，这在全球一级是可以预期的。该项目将展示家庭不平等与当国家再分配与全球公平偏好不匹配时的平等情况相比，家庭不平等如何影响SCC。我计划在国家一级引入三个具体的再分配机制。首先，国家一级通过指挥和控制手段实施减排，而不进一步补偿家庭的政策成本或气候损害。第二，国家治理水平在家庭之间重新分配，以最大限度地发挥国家社会福利功能。从国家治理的角度来看，分配造成的不平等程度是有效的，反映了它对公平的偏好。在第三种情况下，国家层面通过平等的人均方案循环使用碳税收入。该项目的分析部分将揭示在家庭不平等和国家分配政策下SCC是如何变化的。此外，我将使用一个数字综合评估模型来估计通过分析确定的影响的量化范围，该模型解释了经经验校准的不平等程度。我的研究目标是通过包括家庭层面的不平等、将规范的气候变化经济学与国家治理层面的再分配的制度能力约束相结合，来推进关于SCC的科学文献。这一结果对于有关碳排放限额交易体系中最优碳税或最低碳价格水平的政策辩论也很重要。

The social cost of carbon (SCC) has been a central metric for climate policy. The SCC measures the monetary climate damage that is avoided when carbon emissions are reduced by one unit. The level of the SCC critically depends on normative assumptions about justice: global preferences for equity can significantly increase the SCC if climate damages to poor households are not compensated through redistribution from rich households. However, most studies that estimate the SCC assume the availability of efficient instruments for redistribution that balance household inequality in income and climate damages. The SCC may gain importance for regulatory analysis in the future to economically justify ambitious climate policy measures. To increase the robustness of SCC estimates, their derivation needs to take account of real-world constraints to distribution and empirical levels of inequality. This project develops a novel two-level governance model for computing the SCC that allows combining social preferences for equity with inequality at the household level. At the global governance level, the SCC is determined as the optimal carbon tax from maximizing a social welfare function. The national governance level redistributes between households. National transfers lead to a certain level of household inequality that is anticipated at the global level when the SCC is determined. The project will show how household inequality influences the SCC compared to the equality case when national redistribution does not match global preferences for equity. I plan to introduce three specific mechanisms for redistribution at the national level. First, the national level implements emission reductions through command and control instruments without further compensating households for their policy costs or climate damages. Second, the national governance level redistributes between households to maximize a national social welfare function. The level of inequality that results from distribution is efficient from the perspective of the national governance level and reflects its preferences for equity. In the third case, the national level recycles carbon tax revenues through an equal per-capita scheme. The analytical part of the project will reveal how the SCC changes under household inequality and national distributional policies. Additionally, I will estimate the quantitative scope of the analytically identified effects using a numerical Integrated Assessment Model that accounts for empirically calibrated levels of inequality.The goal of my research is to advance the scientific literature on the SCC by including inequality at the household level, combining normative climate change economics with institutional capacity constraints for redistribution at the national governance level. The results are also important for the policy debate about the level of optimal carbon taxes or minimum carbon prices in cap-and-trade systems.