Marine fuel choices and related life-cycle environmental impacts under global shipping policies

Shipping is a transnational sector competing for energy on a global market and needs effective policies to decarbonize. Here we assess energy transitions under different shipping policies using a sector-coupled global energy system model and connected life-cycle environmental impacts. We find that postponed International Maritime Organization’s ‘net-zero framework’ could increase biofuel use in shipping to 40% by 2050 but might fall short of net-zero targets, which require broader adoption of low-carbon fuels. A combined levy and global fuel standards mechanism accelerates low-carbon fuel adoption, achieving 60% of ammonia penetration as marine fuel. Ammonia emerges as most cost-effective fuel under cross-sectoral competition for several assessed policies, whereas electrification is preferred for vessels with daily bunkering possibilities. To reach global climate targets, ammonia from bioenergy with carbon capture (yielding negative emissions) and e-ammonia are found potentially crucial for shipping. However, this transition risks increasing other environmental impacts, including land use and mineral resource depletion.