Carbon leakage in emissions trading systems: a systematic review and meta-analysis of ex-ante and ex-post evidence

Emissions Trading Systems (ETSs) are a widely adopted policy tool for reducing greenhouse gas emissions. However, emission reductions achieved within ETSs can be partially offset by increases in emissions elsewhere , a phenomenon known as carbon leakage. This paper systematically reviews the global literature on carbon leakage associated with ETSs through a combined quantitative meta-analysis and qualitative synthesis. Findings show that ex-ante modelling studies generally report positive carbon leakage rates, with estimates varying widely across schemes. Leakage rates are typically lower when more of the global emissions are covered, anti-leakage measures are included, and when using a computable general equilibrium (CGE) model. Ex-post studies find leakage through channels such as interstate electricity trade in the U.S. and investment relocation in the EU ETS and China pilot ETSs. Findings on goods trade and carbon transfer within firms are mixed. These findings highlight the differences in results between ex-ante and ex-post studies, the heterogeneity of leakage outcomes across schemes, and the critical role of country-specific context in designing ETSs and associated anti-leakage measures.
Key policy insights
The systematic review finds that ex-ante studies estimate a lower leakage rate when emissions coverage is broader, anti-leakage measures are included, and computable general equilibrium models are used.
Ex-post studies evidence leakage through specific channels including investment leakage in the EU ETS and Chinese pilot ETSs, and coal-to-gas substitution in the U.S. regional ETSs.
Jurisdictions designing a mitigation policy should account for context-specific leakage risks when evaluating alternative policy instruments. Further ex-ante evidence on leakage is needed for emerging ETSs beyond the major schemes.
To comprehensively address carbon leakage, a broader, integrated policy mix incorporating both demand- and supply- side anti-leakage measures, enhanced international cooperation, and sector-specific supports is recommended.

The systematic review finds that ex-ante studies estimate a lower leakage rate when emissions coverage is broader, anti-leakage measures are included, and computable general equilibrium models are used.
Ex-post studies evidence leakage through specific channels including investment leakage in the EU ETS and Chinese pilot ETSs, and coal-to-gas substitution in the U.S. regional ETSs.
Jurisdictions designing a mitigation policy should account for context-specific leakage risks when evaluating alternative policy instruments. Further ex-ante evidence on leakage is needed for emerging ETSs beyond the major schemes.
To comprehensively address carbon leakage, a broader, integrated policy mix incorporating both demand- and supply- side anti-leakage measures, enhanced international cooperation, and sector-specific supports is recommended.

排放交易体系（Emissions Trading Systems, ETS）是当前被广泛采用的温室气体减排政策工具。然而，排放交易体系内部实现的减排量，可能会被其他地区的排放增量部分抵消，这一现象被称为碳泄漏。本文通过定量元分析与定性综合相结合的方式，对全球范围内与排放交易体系相关的碳泄漏研究文献进行了系统性综述。研究结果显示，事前模型研究普遍报告存在正的碳泄漏率，且不同体系的泄漏率估算结果差异极大。当全球排放覆盖范围更广、纳入防泄漏措施，以及使用可计算一般均衡（CGE）模型时，泄漏率通常更低。事后研究则发现了多种泄漏渠道，例如美国的州际电力贸易，以及欧盟排放交易体系（EU ETS）和中国试点排放交易体系（China pilot ETSs）中的投资转移。关于货物贸易与企业内部碳转移的研究结论则存在分歧。上述研究结果凸显了事前与事后研究结果的差异、不同体系间泄漏结果的异质性，以及国别特定背景在设计排放交易体系及相关防泄漏措施中的关键作用。

关键政策洞察
本系统性综述发现，事前研究表明，当排放覆盖范围更广、纳入防泄漏措施，且采用可计算一般均衡模型时，估算得到的泄漏率更低。
事后研究证实了特定渠道的碳泄漏，包括欧盟排放交易体系（EU ETS）与中国试点排放交易体系（China pilot ETSs）中的投资泄漏，以及美国区域排放交易体系（U.S. regional ETSs）中的煤改气替代行为。
制定减缓政策的辖区在评估备选政策工具时，应考虑特定情境下的泄漏风险。针对主流体系之外的新兴排放交易体系，仍需更多关于碳泄漏的事前研究证据。
为全面应对碳泄漏，建议采用更广泛的综合政策组合，纳入供需两侧防泄漏措施、强化国际合作，并提供行业专属支持。

本系统性综述发现，事前研究表明，当排放覆盖范围更广、纳入防泄漏措施，且采用可计算一般均衡模型时，估算得到的泄漏率更低。
事后研究证实了特定渠道的碳泄漏，包括欧盟排放交易体系（EU ETS）与中国试点排放交易体系（China pilot ETSs）中的投资泄漏，以及美国区域排放交易体系（U.S. regional ETSs）中的煤改气替代行为。
制定减缓政策的辖区在评估备选政策工具时，应考虑特定情境下的泄漏风险。针对主流体系之外的新兴排放交易体系，仍需更多关于碳泄漏的事前研究证据。
为全面应对碳泄漏，建议采用更广泛的综合政策组合，纳入供需两侧防泄漏措施、强化国际合作，并提供行业专属支持。