Leveraging Earth Observation for Maritime Emission Monitoring: Insights from the EO4SEM Project

Maritime transport underpins nearly 90% of global trade and constitutes a persistent and growing source of anthropogenic greenhouse gas (GHG) emissions, contributing approximately 3–4% of global CO₂ emissions and around 14% of transport-related emissions within the European Union. As global warming alters atmospheric composition and feedback processes in marine and coastal systems, improved monitoring of maritime emissions is increasingly important for constraining warming-induced greenhouse gas emissions (WIE).Funded by the European Space Agency, the EO4SEM project assesses the capability of Earth Observation (EO) technologies to provide independent, spatially explicit information on maritime GHG emissions and to complement bottom-up, activity-based inventories. EO4SEM develops a multi-scale Representative Dataset that integrates satellite observations from TROPOMI, PRISMA, EnMAP, and GHGSat with high-resolution emissions simulated using the Ship Traffic Emission Assessment Model (STEAM v4). Modelled emissions are derived from Automatic Identification System (AIS) data and ship technical characteristics, providing hourly, gridded and ship-level estimates of CO₂, NOₓ, and CH₄ emissions across European maritime regions. Satellite data are analysed using a combination of regional to local atmospheric inversion techniques and plume-based methods adapted for moving point sources, enabling emission estimates at regional, shipping-lane, and individual vessel scales. Regional inversions assimilate TROPOMI NO₂ and CH₄ products into atmospheric transport models to derive monthly emission budgets over major European sea regions; estimates for shipping lanes rely on the analysis of the divergence of mass fluxes in satellite images, while ship-level approaches exploit plume divergence and cross-sectional flux methods to quantify instantaneous emissions from isolated vessels. Hyperspectral data from PRISMA and EnMAP are further explored to evaluate methane detection capabilities in port and coastal environments, and GHGSat glint-mode observations are used to investigate methane emissions associated with liquefied natural gas (LNG) ship-to-shore transfers. Together, these approaches demonstrate the potential of EO to identify emission hotspots, characterise spatial emission patterns, and support independent verification relevant to regulatory frameworks such as the EU Emissions Trading System and Monitoring, Reporting, and Verification requirements. However, EO4SEM also highlights substantial scientific and technical challenges. These include low signal-to-noise ratios and strong background interference in coastal and industrial regions, difficulties in separating ship plumes from land-based sources, and the limited spatial and temporal coverage of hyperspectral sensors. Validation remains a challenge due to scarce in-situ measurements over marine environments and uncertainties associated with incomplete or missing AIS data.The EO4SEM project aims to showcase the transformative potential of EO-based monitoring systems in supporting regulatory compliance, informing policy decisions, and advancing scientific understanding of maritime emissions. Its findings will contribute to paving the way for future Copernicus Sentinel Expansion missions, such as CO2M and CHIME. To support transparency, reproducibility, and policy uptake, all EO4SEM-derived datasets including satellite products, inversion outputs, and model-based emission inventories are being made available through the ESA APEx and geospatial explorer platforms.