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衡量的是碳排放每减少一个单位所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：如果对贫困家庭的气候损害没有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以显著增加SCC。然而，大多数估计SCC的研究都假设存在有效的再分配工具，以平衡家庭收入不平等和气候损害。SCC可能会在未来的监管分析中变得重要，从而在经济上证明雄心勃勃的气候政策措施是合理的。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和经验水平不平等的约束。本项目开发了一种新的两级治理模型，用于计算SCC，该模型允许将社会对公平的偏好与家庭层面的不平等相结合。在全球治理层面，从社会福利功能最大化的角度出发，确定了SCC为最优碳税。国家治理层面在家庭之间进行再分配。国家转移导致一定程度的家庭不平等，这是在确定SCC时在全球一级所预期的。该项目将展示在国家再分配不符合全球公平偏好的情况下，家庭不平等与平等情况相比如何影响SCC。我计划在国家一级介绍三种具体的再分配机制。首先，国家层面通过命令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，国家治理层面在家庭之间进行再分配，以实现国家社会福利功能的最大化。从国家治理层面来看，分配导致的不平等程度是有效的，反映了国家对公平的偏好。在第三种情况下，国家层面通过人均平等方案回收碳税收入。该项目的分析部分将揭示SCC在家庭不平等和国家分配政策下如何变化。此外，我将使用一个数字综合评估模型来估计分析确定的影响的定量范围，该模型考虑了经验校准的不平等水平。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制度能力约束相结合，来推进SCC的科学文献。研究结果对有关“限额与交易”体系中最佳碳税水平或最低碳价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.