The maritime shipping industry is responsible for around 3 % of global greenhouse gas (GHG) emissions, and is now under pressure to decarbonise. Until now, the shipping industry’s main regulator, the International Maritime Organisation (IMO), has failed to implement sufficient policy instruments to support the emission reduction targets set in its Initial GHG Strategy introduced in 2018, and the implemented policy mix lacks consistency and comprehensiveness. This could be considered surprising given that shipping is one of few sectors with a main, global regulatory body, which should allow for a comprehensive and consistent policy mix. This is the puzzle examined in this paper. We identify three main challenges for more consistent, comprehensive and stricter regulation of GHG emissions from international shipping: (1) lack of capacity within the IMO to regulate multiple and emerging technologies, (2) uncertainty around the IMO’s regulatory mandate, and (3) lack of political consensus during negotiations. If the IMO is to play a more progressive role in mitigating climate change, these factors needs to be considered to ensure that the combination of policy instruments are able to achieve set emission reduction targets.