Improving carbon intensity indicator performance for sustainable shipping compliance

Global shipping faces increasing pressure to reduce carbon emissions, particularly under the International Maritime Organization’s (IMO) Carbon Intensity Indicator (CII) framework. This study systematically reviews carbon emission reduction policies and the CII regulatory system while developing an expanded speed optimization model to assess CII compliance under real-world operational constraints. Through numerical simulations, we evaluate how CII responds to key operational parameters, yielding critical insights: (1) Expanding Sulfur Emission Control Areas (SECAs) by 15% increases optimal speed by 2%, raising CII by 0.6%—highlighting unintended trade-offs between sulfur and carbon regulations; (2) Increasing total sailing distance by 5% elevates speed requirements by 5%, but CII worsens by 8%, disproving the assumption that distance alone improves ratings; and (3) Port congestion (e.g. 10% longer port time) forces 12% higher speeds, spiking CII by 23%, while fleet expansion (e.g. 11 to 12 vessels in this study) can reduce CII by 15.8%. These findings demonstrate that liner companies must prioritize integrated strategies—such as route optimization, speed adjustments, and fleet deployment—over isolated measures. This study not only advances the discourse on maritime decarbonization but also equips industry stakeholders with actionable levers to balance compliance, cost, and environmental goals.