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expert reaction to sulphur reductions in shipping fuel and increased maritime warming

A study published in Communications Earth & Environment looks at shipping emissions and maritime warming. 


Dr Laura Wilcox, Associate Professor at the National Centre for Atmospheric Science (NCAS) in the University of Reading, said:

“This is a timely study, but it makes very bold statements about temperature changes and geoengineering which seem difficult to justify on the basis of the evidence.

“The study presents a thorough evaluation of the cloud and radiation responses to the reduction in sulphur dioxide emissions from shipping in 2020, before using a simple mathematical calculation to estimate an associated global warming. It claims that the reduction in the sulphur content of shipping fuel will cause the 2020s to be much warmer than expected, and that the warm global temperatures observed in 2023 are consistent with the warming due to shipping emission changes.

“The upper and lower bounds of the expected warming are within the range of the interannual variability of global mean temperature, but this is not discussed. There seem to be some mathematical issues with the key calculation, as the calculated warming from the energy balance model is applied on top of the observed 2020 temperature anomaly, likely ‘double counting’ some of the warming effect of sulphur reductions. The authors are using 2020 observations as their baseline starting point, which already includes some of the effect that are trying to simulate. This placement of the calculated warming is necessary for the conclusion that the warming effect from shipping sulphur reductions is consistent with the observed temperature anomaly in 2023 – without it, it’s not possible to make this claim.

“Estimates of the radiative forcing due to the 2020 changes in shipping emissions are consistent across models: on the order of 0.1W/m2. The value of 0.2W/m2 presented in this study is averaged over the global ocean only, so is consistent with other estimates.

“An energy balance model is a simple way to estimate a global temperature response to a global radiative forcing. It doesn’t take in to account the pattern of the radiative forcing or any feedbacks, which would be needed to provide a more robust figure on which firmer conclusions could be made.

“The language used in the paper, which is reflected in the press release, frames the change of a shift from high-sulphur emissions from shipping to lower-sulphur emissions as “inadvertent geoengineering”. For many people, a shift to a lower-sulphur shipping fuel that causes less air pollution and reduces aerosol emissions is a move away from human-induced impacts on climate, as well as a move that cuts health impacts from air pollution. Describing this as accidental geoengineering, and presenting figures which may overestimate the impacts, could lead to misguided assumptions about policies intended to curb future emissions.”


Prof Stuart Haszeldine, Director of the Edinburgh Climate Change Institute at the University of Edinburgh, said:

Marine fuel sulphur

“This new analysis by Yuan strongly supports the alarm on global heating due to termination of sulphur aerosol emissions, raised by climate guru James Hansen in 2023 (Global warming in the pipeline. Oxford Open Clim Chan 2023; 3(1):kgad008,

“Sometimes doing the right thing to decrease the effects of emissions, can produce the wrong answer.”


How does this study work?

“The important logic is that SO2 aerosols emitted by combustion of low grade marine diesel can act to reflect solar heat – and so can cool the earth’s surface. This cooling effect is well understood – and documented episodes have occurred as consequences of several major volcanic eruptions emitting SO2 during the past 2000 years. The linkage of heat reflection cooling to clouds at low level above the oceans is modelled using the `Twomey effect’ where cloud particle size alters the reflectivity brightness of cloud. Large droplets in cloud are grey and poor reflectors of heat. Small particles are bright and are good reflectors. Models used in this article assume a linkage of emissions from shipping to cloud reflectivity, which is established from two independent sources. And reflectivity change is fed into an established NASA climate model, to derive regional changes in heat absorbed by oceans. This finds changes of aerosol thickness of 10-40% in the geographic sectors of ocean most affected by shipping. The logic is clearly explained and uses established work in a novel method, which could be replicated by others, to confirm the results. The modelling is very credible, but not mainstream, and still with wide ranges of uncertainty.”



“Using the simple and direct method of heat balance input and heat reflected output, this paper calculates the extra heat retained to be 0.24 K/decade, which is more than double the average warming rate since 1880. This is a huge change from previous modelling. According to very similar prior modelling work by Hansen et al (2023), extra heat if uniformly spread globally will place earth on a pathway towards 8C warming by 2100. That is at and beyond the highest range of conventional IPCC models. Yuan et al concede that strong regional and seasonal variations may exist between heating of different ocean basins. And that the physics of droplets in clouds are still imperfectly understood. Exact predictions should be cautious, although the authors’ trend is very clear, extremely worrying, and very significant.  Other climate scientists (Michael Mann) dispute results that instrumental measurements support the recent uptick in rapid global warming.  As usual, longer time series of more spatially resolved data would be useful. But Yuan et al argue present data is enough to be worrying and to greatly accelerate R&D on mitigating the effects of sulphur aerosol termination.”


What’s at stake?

“Instead of keeping the world on a path to decrease global heating, the removal of SO2 cooling aerosols since 2010 (modelled by this paper since 2020) could place the world on a path to 8C average warming by 2100. Thats a suicide path of temperature for civilisations in temperate and tropical climates, leading to dangerous weather and unpredictable shortages of farmed food.  So these predictions, and expected regional global variability, deserve more than usual attention.”


What to do?

“Cooling the world can restore the predictable seasons farmers need to grow food. Cooler oceans will limit thermal energy powering super storms damaging coastal cities. And the temperate climates of Europe, China, and North America can be protected from heatwaves exceeding 50C before 2050. The technique of Marine Cloud Brightening is already invented. That uses seawater salty spray to create white clouds across oceans, which reflect solar heat, using very little energy or money.”


Doing nothing?

“What if this modelling is correct? Human civilisations are badly prepared for rapid and dangerous climate changes simultaneously around the globe. Yuan et al look to the future, defined by recent measurements and climate models. Hansen et al look to the geological past for answers which do not rely on modelling.  Atmospheric CO2 was last at 2024 levels of 420ppm, 100,000 years ago. With no cooling by sulphur aerosols, land ice melted, and sea level was 6 to 8 metres higher than now. If these studies are correct that would mean rebuilding at least 12 of the worlds megacities on higher ground – often many tens of kilometres inland from today’s coastline.”


Is this GeoEngineering?

“Many environmental scientists are against interventions with climate, because the result are hard to predict, and the interventions need to continue to avoid rapid warming. Yuan et al assert that humans have been inadvertently GeoEngineering sulphur aerosols into the atmosphere from marine diesel fuels. The Geo-Engineering experiment has already been running for many decades. Stopping the SO2 emissions has terminated the experiment, leaving the world average temperature to rapidly ‘catch up’ to where it should have been.  The best remedy is to stop the cause, and stop CO2 and other greenhouse gases entering the atmosphere. 

“Ignoring this warming possibility means relying on perfect energy policy, unprecedented global co-operation, good luck, and the errors of others.”


Dr Shaun Fitzgerald FREng, Director, Centre for Climate Repair, Cambridge, said:

“This is an important paper and provides more evidence about the effect that shipping has been having on the climate for a long time – the CO2 from the funnels has been warming and the sulfur has been offsetting this in part with a cooling effect. The fact is that we need to think very hard about how to navigate our way to a sustainable climate. On 19 Oct 2023 the founders of Extinction Rebellion issued a blog post calling for a review of the IMO regulations fearing that the sudden removal of sulfur might indeed lead to warming of the oceans. A more nuanced approach may be more appropriate; should the sulfur regulations be imposed near shore where the health impacts of emissions have greater impact, but ships be allowed to burn cheaper sulfur containing fuel in the middle of the ocean to help with a cooling effect? Given the evidence in this paper, we need to accelerate development of alternative ways of providing cooling such as Marine Cloud Brightening. This would then enable us to turn to reduced and ideally zero emissions from our maritime fleet without causing sudden increases in temperature.”


Dr Joel Hirschi, Associate Head of Marine Systems Modelling at the National Oceanography Centre, Southampton, said:

“The study shows that the reduction of sulphur in ship fuel since 2020 is likely to accelerate the warming of the planet. This is because the emission of sulphur dioxide (SO2) acts as  “brake” on global warming. It reflects some of the sun’s radiation back into space and when burned it produces fine particles (“aerosols”) which favour the formation of clouds which reflect shortwave radiation from the sun back into space.  This is correct and in line with current knowledge.

“The study also shows that the warming pattern linked to the reduced SO2 emissions is very heterogenous with the strongest warming expected along the busiest shipping routes.  This is a plausible result.

“The authors also say that the emission of SO2 can be regarded as “inadvertent” geoengineering and that the temperature increase seen since 2020 is a proof of its effectiveness.  This is basically correct, even if this study likely overestimates the effect on global temperatures.

“The paper also claims that a large part of the record temperatures of 2023 can be explained by the reduced SO2 forcing.  I didn’t find this claim well supported by the results.

“The estimates of how much the reduced SO2 emissions from shipping would increase the radiative forcing of the planet fall within other estimates I have seen from other authors.  The main caveat is that the study was not conducted in a coupled climate model but with a simple energy-balance model which does not include ocean-atmosphere feedbacks.  These feedbacks would be key to explain the observed global temperatures.  The authors acknowledge this caveat.

“However, I think the authors overestimate the impact the reduction of sulphur in ship fuel had on the global temperatures we witnessed in 2023 and now 2024.  The reason for that is best illustrated in Figure 3 of the paper. The temperature projection obtained by the authors suggests an abrupt shift in the temperature baseline in 2020. However, the observations from 2021, 2022 and 2023 show that the observed values fell markedly below the 2020 level in 2021 and 2022.  Only the record warm year of 2023 falls within the author’s projection.  Starting the projection from the trend line in 2020 would have been a fairer assessment of the impact on global temperatures (rather than starting from a peak in 2020 which was largely due to an El Niño event). In this case the additional temperature increase would be about 0.05-0.1C by 2030 which is not enough to explain the temperatures anomalies we are currently observing.

“The record high temperatures we have been witnessing in 2023 and 2024 are remarkable and they cannot be explained by a single factor.  Research into why recent temperatures have been so high is ongoing and the reduced sulphur content in ship fuel is only one contributing factor.”


Prof Richard Allan, Professor of Climate Science, University of Reading, said:

“Tiny aerosol pollution particles reflect and absorb sunlight as well as making clouds more mirror-like and this shading effect at Earth’s surface has offset some of the heating from rising greenhouse gas amounts that are also caused by human activities. Since the 1980s this cooling influence has waned as cities and countries have begun cleaning up these toxic fumes, uncovering the full influence of greenhouse gas warming. The new research highlights the much discussed, more rapid and globally widespread warming influence of cleaning pollutant particles from ship exhaust fumes, particularly since tighter regulations in 2020. Returning pristine maritime air means the dark ocean surface can readily absorb extra rays of sunlight.

“The study is not the last word and there are varying estimates of how potent this marine cleaning influence has been. The newly estimated heating effect is also only part of the story since it cannot explain on its own the observed increasing rate of heat flooding into the Earth’s climate system since 2000, nor how much the sea surface warmed during 2023 which is caused my multiple factors including natural ocean fluctuations.

“A larger heating effect is identified in the busy shipping region of the North Atlantic which does coincide with elevated ocean temperatures in this region. The uneven heating effect across the globe, that is greatest over northern oceans, could also plausibly affect weather patterns including the location of the wettest monsoons and driest regions, though this is yet to be confirmed.

“The current warming effect just from the shipping regulations is estimated to be even larger than the overall global warming rate since 1980 without this effect. The calculations are also fraught with difficulties in working out precisely how tiny particles affect cloud brightness and how long they last.

“However, the new findings do add still more urgency in massively and rapidly cutting greenhouse gas emissions that are the root cause of the ongoing warming of climate and the intensification of hot, dry and wet weather extremes.”


Dr Hugh Coe, Professor of Atmospheric Composition, University of Manchester, said:

“Clouds have a profound effect on the climate system since they reflect incoming solar radiation back to space that would otherwise be absorbed by the world’s oceans.  Pollution increases the number of particles on which cloud droplets form and this increases the reflectivity of the clouds, enhancing their cooling effect, which has offset some of the warming due to CO2 over the previous decades.  It has been argued that deliberate marine cloud brightening (MCB) by spraying natural sea spray into the atmosphere is a way of reducing climate warming and, though not a substitute for reducing CO2, could be deployed to delay warming until emissions are reduced and/or methods of atmospheric CO2 removal are developed. IMO regulated reductions in sulphur emissions with the prime purpose of improving air quality have reduced sulphate particles above the world’s oceans since 2020. 

“This paper uses model simulations to show the extent to which these reductions have brightened clouds and they calculate that these changes have contributed to 80% of the observed increases in heat retained in the Earth system over the period.  The authors make a strong case that the IMO reductions in sulphur are responsible for substantial changes to marine cloud properties globally and have had a major impact on global heating since 2020.  The IMO 2020 regulations have, in effect, been an inadvertent reverse MCB experiment on a global scale that has significantly dimmed clouds and led to enhancements in planetary heating.  As such, the paper provides evidence that MCB is likely to be an effective geoengineering intervention to offset further warming from CO2 should emissions reductions not be implemented sufficiently quickly to avoid severe climate impacts.  Clearly any MCB intervention would need to be carried out at a scale that is of the same order of magnitude as, but even larger than the global emissions of sulphur from shipping that have recently been reduced.  The work also highlights the geospatial variations in these effects and cautions that any MCB implementation would need to be considered very carefully to avoid substantial regional climate changes that have a negative impact.”


Dr Edward Gryspeerdt, Royal Society Research Fellow, Imperial College London, said:

“This work supports the idea that aerosols can generate a cooling effect on the climate by modifying clouds. Reducing aerosol emissions, such as by regulating the sulphur content of ship fuel, therefore has the potential to cause a warming. This work suggests a stronger warming effect of shipping regulations than other similar studies, but it highlights the uncertain nature of aerosol impacts on clouds.

“This study finds an additional warming of 0.12K over the 10 years following the regulation (2020-2030), an increase of about 60% above the long-term trend in warming caused by greenhouse gases. As most of this warming is expected in the early 2020s, the impact becomes smaller over time when compared to the warming due to greenhouse gases, which will continue to increase for the foreseeable future.

“This warming contributes to recent temperature anomalies, but it is not the sole cause. Other factors, such as El Nino and increases in greenhouses gases have also affected global temperatures during this period, leading to the record-breaking warming we are currently experiencing.

“This work presents the results as the opposite of a geoengineering-like experiment. There is little debate that aerosols act to cool the climate – although there is a lot of uncertainty as to how large that cooling effect is. There remain large uncertainties around the side effects of geoengineering (such as changes to rainfall) and the risks that these changes bring with them. These risks must be better understood to make informed decisions about any future geoengineering strategies.”


Dr Karsten Haustein, Climate Scientist, Leipzig University, said:

“The paper does offer a quantitative estimate of the global and regional effects of the reduction of shipping emissions in 2020 (referred to as  IMO2020 in the paper). Combining observations (satellites) and model simulation (chemistry transport model), the authors were able to estimate a radiative forcing change which is equivalent to a doubling of the background increase in radiative forcing (due to other anthropogenic greenhouse gas emissions) over a time period of approx 7 years. It corresponds to 0.2 W/m2 radiative forcing or 0.16K temperature increase globally (realised over said period of time).

“It’s just one model, but a widely used one and consistent with observational data as far as the microphysical aspects are concerned (e.g. the change in cloud droplet number concentration). Thus, from a quality perspective the evidence they present appears robust. They also discuss the various confounding factors as far as the attribution of the observed warming, especially the record temperatures in 2023, is concerned. Natural variability from El Niño Southern Oscillation as well as the extratropical variability in the North Pacific (coined Pacific Decadal Variability) are mentioned and put in context sensibly.

“They do not claim that all the warming in 2023 is attributable to the shipping sulfate aerosol reduction, but instead provide the above-mentioned theoretical temperature estimate for the global mean in response to the change in radiative forcing. They do not provide such warming estimate for the North Atlantic, yet they do provide a ‘regional’ forcing estimate which could partially explain why the North Atlantic warmed as much as it did in 2023 (0.56 W/m2 basin-wide).

“Given that changes in the atmospheric large scale circulation over the North Atlantic region (associated with a sustained Greenland blocking high pressure system) led to a jet stream shift and reduced wind speeds in early 2023, other factors are clearly at play as well. While the authors did not discuss the circulation changes (which is arguably the only downside of the paper), they refrained from making any strong claim about the North Atlantic warming being caused by the reduced aerosol load. I would have loved to see a quantitative analysis of the effect such wide-spread wind speed reduction in the North Atlantic (be it caused by internal weather variability or indeed somehow dynamically linked to the aerosol reduction), but I’m afraid that remains an open question.

“Since much of the extra warming in 2023 can be linked to the Southern Hemisphere, or circulation changes in SH winter that led to anomalously mild temperatures in Antarctica and the circumpolar ocean region especially in July and September, to be more precise, their estimate of 0.16K warming for the period between 2020 and 2027 may therefore only be a tiny piece of the 2023 warming story. At least this is my interpretation of the results given that others have shown little to no effect of the shipping emission reduction in the North Atlantic region based on other model simulations (yet unpublished but presented at EGU Vienna this year:

“The paper does contribute to understanding why the global radiative imbalance is increasing at the rate it has been increasing lately. That said, last year’s El Niño (and arguably also the wind shift in the North Atlantic region) remains a factor of large uncertainty as far as conclusive statements regarding ‘accelerated warming’ are concerned. The remainder of 2024 will tell us whether or not recent temperature records are indeed something to be even more worried about than the already worrisome pace of our human-made warming during the last few decades. This paper does not provide the final answer yet.”


Prof Matthew Watson, Professor of Volcanoes and Climate, University of Bristol, said:

“Emissions with higher sulfur content have deleterious effects on health including cardiovascular disease and lung cancer. This paper adds to a growing literature looking at climatic impacts of removing sulfur from shipping emissions, by both direct and indirect radiative effects, which indicate that warming has occurred since the legally-mandated reduction of sulfur from shipping emissions in 2020. The paper presents a warming scenario at the high end of a range of estimates that have been recently published. It focuses on observations since 2020 out of necessity, and projections from only a few years’ worth of data are more uncertain. Other factors, such as the Hunga Tonga-Hunga Ha’apai eruption of 2022, large Saharan dust events and El Niño are also likely to have contributed to strong warming since 2020.”


Prof Hamish Gordon, Assistant Professor, Department of Chemical Engineering & Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh USA, said:

“This paper quantifies the additional warming of Earth’s climate that results from 2020 regulations on shipping emissions. The authors find this warming to be 0.16 degrees C, but with at least a factor two uncertainty, and they find most of the warming is expected to happen within about 10 years. A new machine learning method is used in the modeling of cloud droplet concentrations, which is innovative and exciting to aerosol modelers but may also add uncertainty to the results. Another important uncertainty arises from the challenge of quantifying how air pollution affects cloud cover. Despite the large uncertainties, the study highlights the important role of air pollution in influencing global climate.”


Prof Hugh Hunt, Deputy Director, Centre for Climate Repair Cambridge, said:

“The work addresses the science very well, backed up by measured data and modelling.  The conclusion is robust, that since sulphur was removed from shipping fuels since 2020 there has been a notable warming of the Earth, especially of the oceans.  This is due to the cooling effect of sulphur, mainly through the nucleation of clouds which reflect sunlight.  The evidence for this is effect is strong, and the present study explains most of the observed temperature anomalies in recent years.  Of course this is complicated by other effects such as La Nina and El Nino. 

“Implications moving forward, do we put the sulphur back?  No, I doubt that would be acceptable given that removal of sulphur is a highly laudable air-quality initiative.  But alternatives, such as sea-salt aerosol is a very promising alternative.  This is the field of “marine Cloud Brightening” and goes under the umbrella of “Geoengineering” or “Solar Radiation management (SRM” or “Climate Repair”.

“As the title suggests, this could be viewed as an illustration of the “Termination Shock” – what would happen if we stopped doing geoengineering?  Indeed this is a correct perspective, but it mustn’t therefore be interpreted as a reason not to do SRM.  In fact it’s the opposite.  We have been emitting CO2 in such vast quantities and our understanding of climate warming has been adjusted to fit observations.  But we’ve aways had sulphur in our emissions.  Now that they are gone it’s like looking out of the window in the morning daylight after a storm and seeing the damage.  Closing the curtains doesn’t fix anything.

“I am certain that we need some form of cooling.  Emission reduction alone is no longer sufficient.  Carbon capture and storage is necessary too, but we’re progressing too slowly with this.  Arctic summer sea ice will soon be gone. Glaciers are under threat.  Wild fires, droughts, floods, crop failures, biodiversity loss – they’re all on the increase.  We haven’t got much choice.  Marine cloud brightening is an easy one to try out and it looks to be a like-for-like substitute for the sulphur that we are now missing.”


Prof Stephen Turnock, Professor of Maritime Fluid Dynamics, Civil, Maritime & Environmental Engineering, University of Southampton, said:

“From a maritime engineering perspective, removing sulphur emissions after combustion of marine diesel of heavy fuel oil in a ship engine substantially improves air quality for those living downwind often in major port cities such as Southampton. The IMO mandated measure to clean up ship fuel took many years to agree, with the delay causing a legacy of poor health for many. It is interesting to see that the step change while giving greater insight into the complexities of global warming should not be taken as an excuse to rescind the urgent need to clean up shipping’s emissions. The research should impress on the shipping sector how important it is that it accelerates even faster its green transition away from current fossil fuels that both degrade atmospheric air quality and emit large amounts of greenhouse gases.”



‘Abrupt reduction in shipping emission as an inadvertent geoengineering termination shock produces substantial radiative warming’ by Tianle Yuan et al. was published in Communications Earth & Environment at 4pm UK time on Thursday 30 May.





Declared interests

Matt Watson: I have no conflicts of interest

Karsten Haustein: No conflict of interest

Edward Gryspeerdt: I am a coauthor on a related study with the lead author on this paper

Stephen Turnock: I have no conflict of interest

Laura Wilcox: I have no links to the research work for this study, or any research funding which could be considered a conflict of interest. I have worked on research, presented to the European Geosciences Union General Assembly in 2024, that uses a different approach to calculate the climate response to the phasing-out of sulphur from shipping emissions:

Stuart Haszeldine is funded by research grants from EPSRC and NERC with no commercial interests.

For all other experts, no reply to our request for DOIs was received.

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