Marine fuel pathways for net-zero GHG shipping: A review of life cycle assessment evidence and policy alignment

Growing global trade is expected to increase maritime emissions by 50%–250% by 2050 under “business-as-usual” scenario. Decarbonizing the sector therefore requires the adoption of alternative fuels, yet consistent harmonized lifecycle data and alignment with evolving regulations remain limited. This study analyses life cycle assessment (LCA) evidence and policy alignment for marine fuels. It maps and classifies marine fuels according to International Maritime Organization (IMO) defined fuel pathways, harmonizes Greenhouse Gas (GHG) emission and cost data, benchmarks these against IMO and FuelEU regulatory limits, and assesses environmental-economic trade-offs under an evolving regulatory landscape. Consequently, it discusses potential policy-aligned fuel transition roadmaps, challenges in achieving net-zero GHG emissions, and the need for harmonized assessment of marine fuels under IMO pathways. Results show that research predominantly relies on existing infrastructure, technologies, and readily available feedstocks, reflecting a preference for lower-risk transition pathways over transformative energy system shifts. Of the 93 fuel pathways considered in this study, 49 remain unexplored, including 24 of 39 synthetic pathways, while many of the studied pathways are still insufficiently analysed. Only 15 fuel pathways meet both IMO 2035 and FuelEU 2044 targets with only 5 pathways remaining compliant post-2050, highlighting limited mid- and long-term options. Heavy reliance on biofuels faces scalability constraints, while synthetic fuels, though seen as promising long-term scalable solutions, remain largely uneconomical before 2045. Emission–cost trade-offs highlight the need for flexible, multi-fuel transition pathways based on shared characteristics and interoperability to avoid lock-in and support long-term decarbonization.