- Unprecedented marine heat waves in the North Atlantic have been driven in part by a recent drop in shipping emissions, leading to a reduction in highly reflective marine clouds that had previously masked some of the warming from humanity’s greenhouse gas emissions, studies find.
- New limits on sulfur dioxide emissions from shipping, introduced by the International Maritime Organization in 2020, created an inadvertent “natural experiment” that is helping to improve models of the interaction between atmospheric aerosols, clouds and climate.
- The sulfur dioxide emissions reduction also provides the clearest test to date of marine cloud brightening (MCB) — a controversial geoengineering approach proposed to mitigate climate change. It shows that to be effective and avoid a dangerous termination shock, MCB would need to be continuous and sustained.
- Reducing atmospheric aerosol pollution has major benefits for human health, but will also inevitably lead to an unmasking of more dangerous climate warming. This means that improvements in air quality must simultaneously be coupled with decarbonization, experts say.
For many years, the North Atlantic Ocean warmed more slowly than other parts of the world. In 2023, that changed — dramatically.
Over the last year and a half, North Atlantic sea surface temperatures surged, with record-breaking heat extending roughly from Greenland south to the Caribbean, bleaching corals there, while whipping up violent European storms and fueling U.S. hurricanes like Helene and Milton.
The underlying cause of this regionalized heating is undoubtedly greenhouse gas emissions. But while global emissions have been increasingly steadily, the climate has shown an abrupt and dramatic acceleration of warming over the North Atlantic. Many in the scientific community were taken by surprise by the rapid uptick in temperatures there.
After months of analysis, researchers now see several possible factors at play, including climate change and the unusually strong El Niño event that began in June 2023. Another, more controversial explanation for the unprecedented North Atlantic warming seen over the past two years is that it is being driven by a change in shipping regulations, which reduced emissions of a harmful aerosol pollutant into the atmosphere.
This hypothesis hinges on the basic science of how clouds form. Water vapor in the atmosphere can’t form cloud droplets on its own — it needs a tiny particle, or aerosol, to condense around, such as a speck of dust, a grain of sea salt, or a pollutant. Sulfur dioxide emissions from commercial shipping used to create lots of sulfate aerosols, which could seed low-lying, highly reflective clouds, creating long strings of cloud known as “ship tracks” over the ocean. These low marine clouds reflected solar radiation back into space, helping cool the Earth’s climate. This cooling effect was greatest in regions with more shipping.
The current hypothesis: Human-manipulated marine clouds caused by aerosol pollution have helped mask approximately a third of the warming that would have otherwise been caused by greenhouse gas emissions globally. But starting several years ago, those cooling aerosols largely disappeared.
Shipping emissions reductions unmask climate warming
As global shipping has boomed, so has its contribution to atmospheric aerosol pollution — around 13% of global sulfur dioxide emissions come from shipping.
These aerosol pollutants pose a risk to human health, prompting national and international legislation to curb them. First, emissions control zones were introduced in U.S. and European coastal areas. Then, in 2020, the International Maritime Organization (IMO) established new regulations reducing the maximum allowable sulfur dioxide emissions from ship fuel by 80%.
The impact of this dramatic reduction in shipping emissions on marine clouds was felt quickly. Soon after the new IMO regulations came into effect in January 2020, scientists measured a 25% decrease in the number of ship tracks planetwide.
The basic science tells us that fewer marine clouds means less solar energy reflected back into space, and more planet warming. But exactly how much warming this one regulation is responsible for, and where, is still being hotly debated.
With just a few years of data available for analysis, “we are searching for the proverbial needle in a haystack,” says Graham Feingold, a research scientist at the chemical laboratory of the U.S. National Oceanic and Atmospheric Administration (NOAA). This is because “the [warming] signal is small compared to very large variability in the cloud brightness dictated largely by meteorological conditions.”
Several analyses, each using slightly different methods — including computer modeling, satellite imagery, meteorological data and artificial intelligence — have converged on a global estimate of about 0.1 watts per square meter of warming caused by the 2020 IMO shipping emissions regulations. This is equivalent to roughly 3% of the total global warming effect being driven by greenhouse gases.
That’s “a tiny bit of added warming,” says Earth scientist Andrew Gettelman at the Pacific Northwest National Laboratory.
But crucially, that 3% effect was not uniformly distributed across the Earth’s surface. Most of the world’s major shipping corridors are in the Global North, so the warming effect from reduced emissions of ship-based sulfur dioxide has been felt most acutely there. “The effect is small globally, but it’s concentrated over the Northern Hemisphere,” says Gettelman.
He led a recent study that estimated the global and regional effect of sulfur dioxide emissions reductions by comparing the results of computer models that simulated the change in emissions with climate and cloud distribution data from 2022 and 2023. They found that the unprecedented marine heat waves in those years occurred in regions with major shipping routes — where emissions reductions due to cleaner ship fuel would have had the greatest impact.
Although there was very little change in cloud cover over the Southern Hemisphere, the team did detect a change in that half of the world along one major shipping route, just off the coast of West Africa. “There’s no reason that should have happened, except if shipping emissions were a cause of the cloud changes,” Gettelman explains.
However, shipping emissions remain only one part of a complex story: The aerosol reductions explain about 10-20% of the warming experienced in the Northern Hemisphere in recent years, scientists told Mongabay. The rest is due to a combination of greenhouse gas emissions, El Niño, and natural variability.
Climate effects lag behind emissions reductions
Although the effect of reduced sulfur dioxide emissions on marine clouds was noticeable almost immediately, the impact on global climate lagged behind. Gettelman estimates that by the end of 2023, Earth had experienced roughly half of the total unmasked warming that we can expect as a result of the 2020 regulatory change. This lagged effect occurs because “it takes time for the ocean to warm in response to suddenly putting more sunlight on it, because it has a lot of thermal inertia,” he explains.
This “unmasking” is likely to level off in about 10 years, says aerosol and climate scientist Michael Diamond, from Florida State University. After that, “it’s just permanent at that level. Which is very different from something like CO2 emissions, [which are] rising year over year,” he says.
Experts expect unmasked warming due to reductions in shipping sulfur dioxide emissions to add somewhere between 0.05 and 0.1° Celsius (0.09-0.18° Fahrenheit) to the average global temperature by 2030, with more of that warming being felt in the Northern Hemisphere.
An inadvertent geoengineering experiment
The recent abrupt reduction in shipping sulfur dioxide emissions offers the clearest test to date of a controversial geoengineering approach known as marine cloud brightening (MCB), which seeks to cool the planet by spraying aerosols like sea salt into the lower atmosphere to seed the kind of low marine clouds that humans have been inadvertently seeding with sulfur dioxide emissions since the dawn of globalization.
“The first thing that has to happen for marine cloud brightening to be viable as a cooling technology is you have to get the cloud property changes that you would expect,” Diamond says. The abrupt change to marine clouds detected after 2020, “is really clear evidence that we have some leverage on the system.”
However, global-scale warming resulting from the IMO regulatory change has proven hard to detect, suggesting that any attempts at MCB would need to be implemented at a much larger scale to make a noticeable contribution to much-needed global warming mitigation.
But it’s not clear that the cooling effect inadvertently produced via shipping emissions could be scaled up to make MCB a workable strategy. “There’s only certain regions where clouds are susceptible” to cloud-brightening efforts, Gettelman explains.
Unlike carbon dioxide, aerosols have a very short lifespan in the atmosphere — they remain there only for a few days before they are brought back to Earth by rain. This means that decreases in aerosol emissions can have a much more rapid climate impact than decreases in carbon emissions. It also means that for an MCB project to work, aerosols would need to be released almost continuously and sustained over the long term to avoid dangerous termination shocks — sudden temperature surges bringing on disruptive climate effects.
Even if MCB could be implemented effectively, critics have highlighted numerous potential issues. Not least of which is the lack of a clear regulatory framework or system of international governance for a technology that would have global effects.
The highly regional impact of marine clouds also underscores another major geoengineering concern: Because oceans are not evenly distributed across the planet, the cooling effect of MCB deployment would also be unevenly distributed. One recent study estimated that the Northern Hemisphere has experienced 0.32 watts per square meter of warming as a result of the 2020 IMO shipping emissions regulation, compared to just 0.1 watts per square meter in the Southern Hemisphere.
Experts warn that creating or exacerbating temperature disparities between the northern and southern hemispheres could have serious effects on regional rainfall patterns. For example, seeding clouds in the southeast Atlantic Ocean could lead to hotter drier conditions in the Amazon Rainforest — which could in turn push the already hotter, drier Amazon past a tipping point, causing rainforest die-off and massive CO2 releases.
“Focusing seeding efforts in specific areas … may generate changes in regional circulation patterns, with unintended consequences,” warns Feingold, who adds: “the potential for ‘winners’ and ‘losers’ is of great concern.” Early in 2024, African nations proposed a solar geoengineering non-use agreement precisely because of the risk of such Global North vs. Global South inequities.
Avoiding dangerous termination shocks
Perhaps the clearest lesson from recent shipping emissions reductions is that, if we initiate marine cloud brightening at a scale large enough to make a real dent in climate warming, then we would probably be locked in to that strategy for years or decades.
Any sudden unexpected end during this hypothetical future MCB project — caused by war, civil unrest, natural disaster, political turnover, or economic downturn, for example — could trigger a termination shock that causes more harm than the climate warming the geoengineering averted.
“The disruptiveness of that termination shock is going to be totally dependent on how much cooling you’re doing,” Diamond says. If humanity was offsetting a great deal of warming caused by carbon emissions through deliberate marine cloud brightening, then suddenly stopped, the effects would be much more immediate and noticeable.
However, if you reduced MCB gradually “in a planned way, you wouldn’t necessarily have a termination shock” but more of a “glide path back to where you would have been before” without MCB mitigation, explains Diamond.
Aerosol emissions reductions are already built in to climate models, including the Shared Socioeconomic Pathway (SSP) scenarios used by the Intergovernmental Panel on Climate Change (IPCC) to project future climate change. However, most climate models anticipate a more gradual decrease in aerosol emissions than those just accomplished by the IMO regulations.
The dramatic shipping emissions reductions since 2020 are part of a larger-scale gradual trend of cutting aerosol emissions, which began more than 30 years ago. What makes recent events different is the speed of the change, which appear to have accelerated global warming in the Northern Hemisphere.
A natural experiment on cloud-climate interactions
The sulfur dioxide pollution reductions resulting from the 2020 IMO regulations “have provided us with a very interesting ‘natural experiment’ to explore the unmasking of greenhouse gas warming,” says Feingold.
The basic science of how aerosols, clouds and climate interact at small scales are well understood, but the complex dynamics of how these interactions might play out over longer time periods and across large expanses of atmosphere is still an active area of research.
“Translating what we have learned at these smaller scales up to the scales that are relevant to [global] climate projections remains significantly more challenging,” says Feingold “An enormous challenge is … to develop climate projections based on models and observations at the full range of atmospheric scales.”
Sulfur dioxide emissions globally are currently masking between 20 and 40% of human-caused warming, Gettelman says. That’s 0.3-0.6°C (0.5-0.1.1°F) of masked warming. So the more sulfur dioxide emissions we cut, the more aerosol cooling we lose, and the more rapidly the planet will warm. But the uncertainty created by that large 20-40% range matters a great deal. If the actual masking effect is at the higher end of this estimate, “it means the termination shock [will be] larger” if big sulfate reductions are accomplished.
Natural experiments, like the one resulting from the IMO shipping sulfur dioxide cuts, can help scientists improve their models of aerosol-cloud interactions and refine estimates of the amount of warming we can expect to be unmasked as aerosol emissions drop.
By looking “at these unintended experiments … we can try to validate our models against the large scale,” says Gettelman.
Coupling decarbonization with pollution cuts
Some argue that making deep aerosol cuts — which will unmask some of the warming caused by greenhouse gas emissions, unleashing more intense and numerous extreme weather events — is a good reason to abandon attempts to cut health-harming air pollution.
But Diamond says this is the wrong way to look at the situation: Rather than viewing air pollution and climate change emissions cuts as two opposing priorities, humanity should understand them as mutually dependent co-benefits.
Put simply, to avoid the danger of increased atmospheric instability, we must act decisively to lower CO2 emissions, methane emissions and aerosol emissions in unison.
“If we want to avoid more of this kind of ‘termination shock’ as we reduce air pollution, that means coupling it with decarbonization,” Diamond says.
Banner image: Sunset and a Maersk container ship. Recent cuts in marine sulfur dioxide emissions resulted in an increase in temperature over the shipping lanes of the North Atlantic Ocean, ending an unintended marine cloud brightening geoengineering experiment. Image by —=XEON=— via Wikimedia Commons (CC BY 3.0).
Cloud brightening over oceans may stave off climate change, but with risk
Citations:
Watson-Parris, D., Christensen, M. W., Laurenson, A., Clewley, D., Gryspeerdt, E., & Stier, P. (2022). Shipping regulations lead to large reduction in cloud perturbations. Proceedings of the National Academy of Sciences, 119(41). doi:10.1073/pnas.2206885119
Zhang, J., Chen, Y., Gryspeerdt, E., Yamaguchi, T., & Feingold, G. (2024). Large radiative forcing from the 2020 shipping fuel regulation is hard to detect. doi:10.21203/rs.3.rs-4552523/v1
Gettelman, A., Christensen, M. W., Diamond, M. S., Gryspeerdt, E., Manshausen, P., Stier, P., … Yuan, T. (2024). Has reducing ship emissions brought forward global warming? Geophysical Research Letters, 51(15). doi:10.1029/2024gl109077
Yuan, T., Song, H., Oreopoulos, L., Wood, R., Bian, H., Breen, K., … Platnick, S. (2024). Abrupt reduction in shipping emission as an inadvertent geoengineering termination shock produces substantial radiative warming. Communications Earth & Environment, 5(1). doi:10.1038/s43247-024-01442-3
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