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Has reducing ship exhaust pollution accelerated global warming?
16 August 2024
A new study explored the climate effect of the mandated reduction of sulphur in ship exhaust emissions globally since 2020. Highly relevant to international policy, the study suggests that the shipping regulation has reduced how much light is being reflected back into space, which has likely contributed towards the record warming over the last few years.
Ship tracks. Image courtesy of the National Oceanic and Atmospheric Administration (NOAA)
International shipping, while invisible to most of us, has a large impact on climate and air quality. There are nearly 100,000 large ships within the global commercial fleet, accounting for over 90% of international trade. Traditionally, ships have burned dirty, high sulphur fuel that emitted large quantities of sulphur gas and aerosol.
How is an aerosol different to a gas?
Particulate matter or droplets suspended in air are called aerosol. Aerosol includes airborne dust, mists, fumes and smoke and can be of primary origin (e.g. dust) or secondary origin (e.g. transformed from gases, including sulphur).
‘Dirty’ ship emissions increase the background sulphur aerosol levels over the ocean. The sulphur-containing aerosol act as cloud ‘seeds’, and condensation of water vapour onto these seeds lead to cloud formation. Not only do sulphur aerosol increase cloudiness but these polluted clouds are also generally brighter, due to the small droplet size of the water vapour, which results in more light being reflected back to space.
Consequentially, ship emissions have likely had an unintended cooling effect on the planet, offsetting some of the warming due to greenhouse gases. However, the magnitude of this cooling effect is poorly known.
Aerosol, especially those containing sulphur, are a principal component of air pollution and can adversely affect respiratory and cardiovascular health. Due to air quality concerns, the International Maritime Organization mandated a 80% reduction in the maximum allowed sulphur emission from global shipping in 2020 (IMO2020) so what is the climate impact of this drastic, large scale policy change?
By using several models, this latest study estimates the change in aerosol induced-cooling as a result of the IMO2020 regulation. The study showed that the spatial patterns in the modelled reduction in cooling strongly correlated with the observed changes in clouds from satellite, with less light reflected back to space. This further correlated with the increased Northern Hemisphere surface temperatures during 2022–2023.
These findings suggest that the reduced sulphur emission since IMO2020, while improving coastal air quality, has accelerated global warming. However, the modelled reduction in cooling due to the regulation can only account for a fraction of the observed cloud changes in recent years, implying that although the shipping regulation contributed, it does not completely explain the record breaking temperatures over the last few years.
Dr Mingxi Yang, an author on the study, Chemical Oceanographer at Plymouth Marine Laboratory and lead investigator on the ACRUISE project that helped support this study, said:
“This study represents our current best estimate of the impact of ship emission changes on climate, which still may be on the conservative side. Aircraft sampling of ships emissions before and after IMO2020, as part of the ACRUISE project, showed that the shipping regulation not only changed the amount of sulphur gas being emitted but probably also made the ship-emitted aerosol less efficient at seeding clouds. However, this aspect is not yet accurately represented in models and thus, the full impact of the ship sulphur regulation on clouds still needs to be refined, which is on-going work. Understanding the impact of IMO2020 on climate is not just relevant for the current decade, but for several decades to come as the world aims to rapidly decarbonize and further reduce aerosol emissions.”
The IMO2020 regulations can be seen as an inadvertent experiment in solar geoengineering, but in reverse as it caused warming. Ship sulphur reductions may have contributed a significant amount to the 2023 extreme temperatures in the Northern Hemisphere, but the estimated magnitude appears to be too small to be the only cause. Events like the significant El Niño-Southern Oscillation (ENSO) warming episode from mid-2023 would have played a role as well.
Dr Edward Gryspeerdt, author on the study and Royal Society University Research Fellow at Imperial College London, said:
“To what extent are we accelerating climate change by cleaning up air quality faster than limiting greenhouse gas emissions? Our inability to provide a robust attribution of global radiative forcing and resulting temperature changes even three years after such a large experiment speaks to the huge challenges in managing any deliberate intervention due to natural variability".
To help refine the estimates of aerosol induced-cooling as a result of the IMO2020 regulation even more in the future, a better comprehension of ship aerosol emissions and access to ship position data is needed. Understanding the risks and benefits of emissions reduction and the difficultly in robust attribution highlights the large uncertainty in attributing proposed deliberate climate intervention.
This study was led by Pacific Northwest National Laboratory (USA) in partnership with scientists from Florida State University (USA), Imperial College London (UK), University of Oxford (UK), University of California (USA), Plymouth Marine Laboratory (UK), University of Leeds (UK) and University of Maryland (USA).
Related information
Full paper: Has Reducing Ship Emissions Brought Forward Global Warming?