Michael Diamond thought he would have to wait until this year to have enough data to understand how a shipping regulation aimed at limiting pollution was affecting the clouds that blanket the ocean. “They’re so variable. They’re so tenuous. They’re changing so constantly,” he said. “So you often really need a lot of observations to know what they’re doing.”

Still, just three years after the global maritime community slashed sulfur emissions in 2020, the Florida State University cloud physicist published a prize-winning paper studying clouds along a shipping lane in the southeast Atlantic. By analyzing satellite data from before and after the regulations went into effect, Diamond showed that the clouds had dimmed. In other words, even as the global shipping industry cleaned up its emissions, he made clouds at sea slightly less bright.

This change has important implications for the planet. It means less sunlight is reflected back into space — which means more warming.

That impact extends far beyond the isolated shipping lane Diamond studied. Others around the world have discovered it in the years since the International Maritime Organization adopted the rule. The regulation, known as IMO 2020, lowered the maximum sulfur content in marine fuels for all ships, container ships and cruise ships from 3.5 percent to 0.5 percent, with the aim of cleaning up the air in ports and the communities around them, potentially saving hundreds of thousands of lives each year.

It worked. Measurably lower levels of ammonia and sulfur dioxide pollute the air around many ports, and most marine fuel tested by the agency meets limits. Still, it has had the unintended consequence of increasing global warming in the short term.

How much warmer it will get remains to be seen.

“I think there will be a lot of papers on reducing shipping emissions,” said Robert Allen, a climate scientist at the University of California Riverside with expertise in aerosols. “I don’t think they’re all necessarily going to argue the same thing.”

Aerosols, short-lived pollutants suspended in the atmosphere, introduce more uncertainty into climate models than any other variable. One of the most common is sulfur. Unlike its carbon cousin, sulfur dioxide cools the planet by creating aerosols that reflect sunlight while brightening clouds. When industries around the world release less of this pollutant, clouds darken. The planet absorbs more sunlight, and the land, air, and water warm even faster than before.

In late May, Tianle Yuan, an atmospheric scientist at NASA’s Goddard Space Flight Center, published one of the first papers using observational data and climate models to determine what this means for Earth. The results were stark and surprising.

So far, temperatures have risen by an average of almost a fifth of a degree Celsius per decade since 1981 — a little over a third of a degree Fahrenheit. Yuan’s results suggest that the sharp decline in ocean aerosols will increase temperatures by another quarter of a degree Celsius over the next decade. “This decade, we expect warming to more than double,” Yuan said, “if our calculations are correct.”

But as is the case in science, not everyone agrees that he and his colleagues were right.

Robert Allen, for example, believes that Yuan and company have gone down the wrong path. He and his colleagues conducted their own study (which is awaiting peer review) and while they agree with Yuan about how the regulations have affected the amount of light the Earth receives, they “came to some different conclusions.”

“We’re going to get less than 0.05 degrees,” Allen said, “over the next 20 years.” In fact, as he and his coauthors noted, their range of results is consistent with “no observable” impact on global temperatures. The discrepancy between the two results, Allen noted, comes down to how they simulated the impact.

At its core, the differences between Allen’s results and Yuan’s come down to a difference in modeling. Yuan relies on an energy balance model, which makes simplifying assumptions about the planet to calculate the temperature change associated with a given forcing on the climate. Allen, on the other hand, used an Earth system model, which attempts to incorporate more realistic representations of the Earth’s climate because it tries to predict how changes in the composition of the atmosphere will affect temperature, among other things.

A third study by two Cornell researchers also used an Earth system model and found results that largely match Yuan’s. The difference here can be explained, at least in part, by the number of “ensemble members” used. Simply put, each ensemble member represents the same model run with slightly different initial conditions, an approach that allows scientists to explore the myriad ways that even small factors can push the climate in different directions. Think of it as a way to try to account for the butterfly effect. A large ensemble, then, allows researchers to separate the signal from the noise and tease out the true impact that produces something like IMO 2020.

Model differences aside, a more modest impact seems more reasonable. As Allen and Diamond both noted, if all the world’s aerosols suddenly disappeared, the planet would warm by at least half a degree Celsius, and at most by just over 1 degree. And while IMO 2020 reduced maritime sulfur emissions by nearly 80 percent, shipping accounted for less than 10 percent of global emissions of the pollutant even before the regulations were passed. That means even deep cuts in that sector would be expected to produce a limited response.

Ultimately, while some suggest warming will double this decade, while others point to only a small increase, Allen said, “The science is not closed.” As a result, the current debate tells us less about the specific effects of cleaning up shipping pollution and more about the potential dangers of eliminating aerosols without also addressing greenhouse gases.

No one doubts that reducing sulfur emissions will benefit public health. A study published in 2016 found that imposing a cap on sulfur emissions in 2020 would prevent at least 570,000 premature deaths over the next five years. But since sulfur also cools the planet, it is at least not possible to reduce short-lived greenhouse gases such as methane while reducing sulfur pollution only accelerates the rate at which the planet warms.

Reducing carbon dioxide is crucial, of course. But it can linger in the atmosphere for a millennium. Aerosols, on the other hand, fall out of the sky within weeks, meaning that the incidental cooling effect they produce doesn’t linger long after their emissions stop — even if carbon in the air continues to trap heat. Fortunately, the greenhouse gases methane and ground-level ozone, when combined, warm the planet just as much as aerosols cool it. And they don’t stick around nearly as long as CO2. Low-level ozone lasts a few weeks at most, and methane disappears within a decade.

So eliminating those pollutants along with aerosols could negate the abrupt warming that might otherwise occur. “But that’s the opposite of what’s happened,” Allen said. Not only has the sulfur spewed by ships all but disappeared, China has also made great efforts to clean up its air faster than expected. Meanwhile, carbon emissions continue to rise worldwide. Yuan and many others argue that this has created a “termination shock,” leading to an abrupt rise in global temperatures with additional effects beyond heat. Eliminating aerosols without a corresponding reduction in greenhouse gases has the potential to exacerbate wildfire activity in boreal forests, slow a vital ocean current and affect regional weather patterns in ways not yet understood.

All of this has prompted seven climate activists and researchers to pen an open letter to the International Maritime Organization . They urge the organization to consider allowing ships to burn dirtier fuel on the high seas, far from population centers, in order to “amplify the global cooling benefits of sulfur or similar aerosols without harming people or natural systems.” They also ask the organization to support research and testing of technologies that would allow ships to, for example, create salt aerosols from seawater and spray them into the air to get the benefits of brighter clouds without the side effects of sulfur.

While a spokesperson for the International Maritime Organization said it welcomes reports and research from almost anyone, it will only consider regulations if they are suggested by member states. Whether the open letter will have any impact, therefore, remains to be seen. The dispatch nevertheless represents an early rumbling of what many scientists fear could become a call to manipulate the planet in ever more dramatic and deliberate ways as the climate crisis deepens.

Such appeals aside, the unintended impact of IMO 2020—not to mention climate change itself—makes clear that humanity has long been manipulating the atmosphere. But the debate between Allen, Yuan and their colleagues raises the fundamental question of how any technologist can hope to achieve a precise degree of deliberate cooling when scientists can’t agree on the precise impact of our unintended experiment.