PNNL

Abstract

Ships have a unique climate effect due to brightening of low marine clouds from emissions of sulfur and aerosols, resulting in visible “ship tracks”. In 2020, new shipping regulations mandated an ~80% reduction in the allowed sulfur emission over the global oceans. Recent observations using new methods indicate that indeed visible ship tracks, and perturbations to clouds from ship emissions in general (‘invisible tracks’) have decreased. Model simulations indicate that the shipping regulation has induced a radiative forcing of about +0.12 Wm-2 starting in 2020. Northern Hemisphere surface temperature anomalies in 2022–2023 are correlated with observed cloud radiative forcing. Changes in clouds are too large to be just feedbacks from surface temperature anomalies, indicating changes in clouds due to factors such as shipping are likely driving at least part of the surface temperature increase. Furthermore, simulations of radiative effects from shipping have a significant pattern correlation with the cloud changes over 2022–2023, indicating a possible role for shipping emissions changes. Energy balance modeling indicates regional changes from shipping might have contributed 30–40% of regional (N. Hemisphere extra-tropical ocean) temperature anomalies in 2023, and this is only half the realizable warming from shipping emissions changes until 2030. To better understand and constrain these estimates, better access to ship position data and understanding of ship primary aerosol emissions would be valuable.