| 26 May 2026
Addressing systemic underestimation in global ship emissions from fleet growth and fuel compliance
Abstract. As a hard-to-abate sector, global shipping is under international and regional emission control regulations. To evaluate emission control effects and conduct rapid response air quality simulations, accurate and timely ship emission inventories are indispensable. However, current ship emission inventory models face multiple challenges, including incomplete and delayed global ship fleet description and significant divergence in PM2.5 emissions after global low sulfur regulation came into effect. Here, we established a dynamically updated Ship Emission Inventory Model that allows near-real-time emission calculation. Ship activity and technical database are updated daily instead of yearly to obtain a more complete and rapid description of global ship fleet. Fleet's multiple choices to comply with fuel sulfur regulation were considered, including switching to very low sulfur fuel and utilizing scrubbers to keep consuming heavy fuel oil. The daily expansion of ship technical database uncovered 8% and 6.2% of the total gross tonnage of active bulk carrier and container fleets, unveiling up to 5.4% of global ship CO2 emissions. Without the expansion, the daily underestimation would enlarge over time from about 0.20 Mt CO2/d to 0.29 Mt CO2/d throughout 2024. On the other hand, the single compliance choice assumption and ignorance of heavy fuel oil use after 2020 would lead to underestimation of PM2.5 and BC emissions by approximately 55% and 27%. Although South China Ocean had the most absolute underestimation, the Indian Ocean had the highest underestimated portion, reaching 75% and 39% of its total PM2.5 and BC emissions.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
This paper present an updated version of a ship activity model to better estimate the atmospheric emissions of CO2, SOX, BC, NOX and PM2.5 where both number of vessels, distance sailed and compliance option with respect to the IMO global sulfur cap is included. While presenting important new data, showing the underestimation of all emissions of atmospheric pollutants I lack a further discussion on 1) if authors think it is the adoption to daily resolution, the data curation and/or the addition of more vessels to the model that have the greatest influence on the results and 2) given the systematic underestimation (and 20% is a lot), what are the implications of the shipping fleet's impact on the envrionment and human health and what can be proposed as measures to improve reporting, data sharing and inclusion og global shipping in future climate/impact models? The authors focus on two compliance options (low sulfur fuels and HFO+scrubber) but given the high temporal resolution of ship activity dat, would it be possible to divide low sulfur options into VLSFO (in 0.5% areas) vs MGO (and ULSFO) (in 0.1% areas)? My guess is that emission factors are different but the question is if data is available to differentiate. On emission factors, would it be possible to present these as ranges instead of absolute numbers to reflect the variability? Is engine load accounted for in the modelling, if not the authors could discuss the implications of including this? For many pollutants, emisison factors are tested at different loads so it should be possible to do some estimations (present ranges as proposed in previous point).Â
Some minor comments: