In late 2016, the International Civil Aviation Organisation (ICAO) made headlines, reaching a long-awaited multilateral agreement on a global market-based measure (GMBM) to regulate carbon emissions from civil aviation. This was hailed as a ‘historic achievement’, with 65 countries responsible for 85% of emissions working together to protect the global climate. Importantly, the speed of negotiations was greatly catalysed by the European Union, which was set to reinstate its own unilateral aviation emissions trading scheme on 1 January 2017 should the ICAO fail to reach an agreement by the end of the previous year (Regulation 421/2014). In the nick of time, at its 39th meeting, ICAO reached an agreement on a global MBM, known as the ‘Carbon Offsetting and Reduction Scheme for International Aviation – ‘CORSIA’. It was clearly hoped that this would prevent the Union from proceeding with its unilateral approach. However, in its recent Proposal for a regulation amending the EU ETS, the Commission appears reluctant to immediately relinquish its leverage, raising political tensions and important legal questions regarding the compatibility of the two schemes.