Surface temperature effects of recent reductions in shipping SO2 emissions are within internal variability

Watson-Parris, Duncan, Wilcox, Laura J., Stjern, Camilla W., Allen, Robert J., Persad, Geeta, Bollasina, Massimo A., Ekman, Annica M. L., Iles, Carley E., Joshi, Manoj, Lund, Marianne T., McCoy, Daniel, Westervelt, Daniel M., Williams, Andrew I. L. and Samset, Bjørn H. (2025) Surface temperature effects of recent reductions in shipping SO2 emissions are within internal variability. Atmospheric Chemistry and Physics, 25 (8). pp. 4443-4454. ISSN 1680-7324

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Abstract

In 2020, the International Maritime Organization (IMO) implemented strict new regulations on the emissions of sulfate aerosol from the world's shipping fleet. This can be expected to lead to a reduction in aerosol-driven cooling, unmasking a portion of greenhouse gas warming. The magnitude of the effect is uncertain, however, due to the large remaining uncertainties in the climate response to aerosols. Here, we investigate this question using an 18-member ensemble of fully coupled climate simulations evenly sampling key modes of climate variability with the NCAR model, the Community Earth System Model version 2 (CESM2). We show that, while there is a clear physical response of the climate system to the IMO regulations, including a surface temperature increase, we do not find global mean temperature influence that is significantly different from zero. The 20-year average global mean warming for 2020–2040 is +0.03 °C, with a 5 %–95 % confidence range of [], reflecting the weakness of the perturbation relative to internal variability. We do, however, find a robust, non-zero regional temperature response in part of the North Atlantic. We also find that the maximum annual mean and ensemble mean warming occurs around 1 decade after the perturbation in 2029, which means that the IMO regulations have likely had very limited influence on observed global warming to date. We further discuss our results in light of other, recent publications that have reached different conclusions. Overall, while the IMO regulations may contribute up to 0.16 °C [] to the global mean surface temperature in individual years during this decade, consistent with some early studies, such a response is unlikely to have been discernible above internal variability by the end of 2023 and is in fact consistent with zero throughout the 2020–2040 period.

Item Type:	Article
Additional Information:	Data availability statement: The underpinning simulation output used in this work is available at https://doi.org/10.5281/zenodo.15185388 (Watson-Parris, 2025). Funding information: This research has been supported by the Vetenskapsrådet (grant no. 2022-06725), the Norges Forskningsråd (grant nos. 324182 and 315195), the Directorate for Geosciences (grant nos. 2235177 and 2153486), and the EU Horizon 2020 (grant no. 821205).
Uncontrolled Keywords:	sdg 13 - climate action ,/dk/atira/pure/sustainabledevelopmentgoals/climate_action
Faculty \ School:	Faculty of Science > School of Environmental Sciences University of East Anglia Research Groups/Centres > Theme - ClimateUEA
UEA Research Groups:	University of East Anglia Schools > Faculty of Science > Tyndall Centre for Climate Change Research Faculty of Science > Research Centres > Tyndall Centre for Climate Change Research Faculty of Science > Research Groups > Centre for Ocean and Atmospheric Sciences Faculty of Science > Research Groups > Climatic Research Unit
Related URLs:	https://acp.copernicus.org/articles/25/4...
Depositing User:	LivePure Connector
Date Deposited:	23 Apr 2025 14:30
Last Modified:	28 Apr 2025 00:21
URI:	https://ueaeprints.uea.ac.uk/id/eprint/99060
DOI:	10.5194/acp-25-4443-2025

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