Carbon-Market Fragmentation after Brexit

After Brexit, UK and EU carbon prices diverged, with a UK discount of about €23/tCO₂ emerging in 2023, creating a natural experiment in carbon-market fragmentation. We examine whether this gap redirected CPC Y02 green-technology patenting in CBAM-exposed sectors: steel, cement, aluminium, fertiliser, and hydrogen. Across seven specifications, the estimated effects are consistently Porter-compatible and economically meaningful. However, none remains significant under randomization inference that properly accounts for a single treated country. The most defensible interpretation is a forward-looking response to expected policy developments—such as a future UK CBAM or potential EU–UK ETS relinking—rather than confirmed evidence of a UK-specific causal effect. More broadly, the findings highlight a key lesson for Brexit and other single-jurisdiction studies: firm-level disaggregation can improve precision, but it does not create additional treated clusters; credible inference remains constrained by country-level permutation tests.英国脱欧（Brexit）后，英国与欧盟的碳价出现分化，2023年两国间形成约23欧元/吨二氧化碳当量（€23/tCO₂）的价差，由此构成了碳市场割裂（carbon-market fragmentation）的自然实验。本研究旨在考察该价差是否使受碳边境调节机制（CBAM, Carbon Border Adjustment Mechanism）覆盖的行业中的CPC Y02类绿色技术专利活动发生转移，涉及行业包括钢铁、水泥、铝、化肥及氢能。在七种模型设定下，估计得到的效应始终与波特假说相符且具备显著经济意义。然而，在恰当考虑单受干预国家的随机化推断框架下，所有效应均不再显著。最具说服力的解释为，企业的响应是针对预期政策动向的前瞻性行为——例如未来英国碳边境调节机制的推出，或是欧盟与英国排放交易系统（ETS, Emissions Trading System）的重新挂钩——而非英国特有的因果效应的确凿证据。从更广泛的视角来看，本研究结论为英国脱欧及其他单辖区研究提供了重要启示：企业层面的细分可以提升估计精度，但无法创造额外的受干预集群；可靠的统计推断仍受限于国家层面置换检验的约束。