Michael Diamond thought he would have to wait at least until this year to have enough data to understand how a shipping regulation aimed at limiting pollution affected the clouds covering the ocean. “They are so changeable. They are so pissed off. They are always, always changing,” he said. “So you really need a lot of observations to figure out what they’re doing.”
Nevertheless, just three years after the international maritime community reduced sulfur emissions in 2020, the Florida State University cloud physicist awarded paper studying clouds along a shipping lane in the southeast Atlantic. By analyzing satellite data from before and after the regulation took effect, Diamond demonstrated this the clouds dimmed. In other words, even as it cleaned up its emissions, the global shipping sector made marine clouds a little less clear.
This change has important implications for the planet. This means less sunlight is reflected back into space – which means more warming.
This effect extends far beyond the isolated shipping lane that Diamond studied. Others have tracked it worldwide in the years since the International Maritime Organization adopted the rule. The regulation, shorthand to IMO 2020reduce the maximum level of sulfur in marine fuel for all ships, container ships and vessels, from 3.5 percent to 0.5 percent with the aim of cleaning 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 the majority of marine fuels tested by the organization meet the limits. Yet this had the unintended consequence of increase global warming in the short term.
How much hotter things will get remains an open debate.
“I think there are going to be a lot of papers on reducing shipping emissions,” said Robert Allen, a University of California Riverside climate scientist with expertise in aerosols. “I don’t think they’re all necessarily going to argue the same way.”
Aerosols, which are short-lived pollutants suspended in the atmosphere, introduces more uncertainty in climate models as any other variable. One of the most generally comes from sulfur. Unlike its carbon cousin, sulfur dioxide tends to cool the planet by creating aerosols that reflect sunlight while also make clouds brighter. When industries around the world emit less of these pollutants, clouds darken. The planet absorbs more sunlight, and land, air and water heat up even faster than before.
At the end of May, Tianle Yuan, an atmospheric scientist at NASA’s Goddard Space Flight Center, published one of the first papers to use observational data and climate models to determine what this means for Earth. The results were stark and startling.
Until now, temperatures have typically risen almost a fifth of a degree Celsius, on average, per decade since 1981 — just over a third of a degree Fahrenheit. Yuan’s results suggest that, over the next decade, the sharp decline in ocean aerosols will cause temperatures to rise an additional quarter of a degree Celsius. “This decade, we expect the warming rate to more than double,” Yuan said, “if our calculation is correct.”
But, as happens in science, not everyone agrees that he and his associates managed to do it.
Robert Allen, for one, believes that Yuan and company have gone astray. He and his colleagues led a study of their own (it awaits peer review), and while they agree with Yuan about how the regulation affected the amount of light Earth receives, they “came to some different conclusions.”
“We’re getting less than 0.05 degrees,” Allen said, “over the next 20 years.” In fact, as he and his co-authors noted, their range of results is consistent with “no discernible” impact on global temperatures. The difference between the two results, Allen indicated, comes down to how they simulated the impact.
At their core, the differences between Allen’s results and Yuan’s boil 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 modelwhich seeks to include more realistic representations of the Earth’s climate, as it attempts to predict how changing the composition of the atmosphere will affect temperature, among other things.
A third study by a pair of Cornell researchers also used an Earth system model and came up with results largely consistent with Yuan’s. The difference here can be explained, at least in part, by the number of “ensemble members” used. Simply put, each member of the ensemble represents the same model run with slightly different initial conditions, an approach that allows scientists to explore the many ways that even small factors can push the climate in different directions. Think of it as a way of trying to account the butterfly effect. A large ensemble therefore enables researchers to separate the signal from the noise and discern the real impact that something like IMO 2020 is delivering.
Modeling differences aside, a more modest impact seems more reasonable. As both Allen and Diamond pointed out, if all the aerosols in the world suddenly disappeared, the planet would warm by at least half a degree Celsius and at most, just over one degree. And although IMO 2020 reduced maritime sulfur emissions by almost 80 percent, shipping was responsible for less than 10 percent of global emissions of the pollutant even before the regulation was adopted. This means even deep cuts in that sector should produce a limited response.
Ultimately, though — with some suggesting that warming will double this decade, while others suggest only a slight increase — Allen said, “the science is not closed.” Consequently, the current debate tells us less about the specific effects of cleaning up ship pollution and more about the potential dangers of eliminating aerosols without also addressing greenhouse gases.
No one doubts that reducing sulfur emissions benefit public health. One study published in 2016 found that introducing the limit on sulfur plating in 2020 prevent at least 570,000 premature deaths over the next five years. But since sulfur also cools the planet, failing to at least short-term greenhouse gases such as methane while reducing sulfur pollution simply increases the rate at which the world is warming.
Cutting carbon dioxide is obviously critical. But it can stay in the atmosphere for a millennium. Aerosols, on the other hand, fall from the air within weeks, meaning that the accidental cooling effect they produce doesn’t stick around long after their emission ends—even if carbon in the air continues to trap heat. Fortunately, when combined, the greenhouse gases methane and ground-level ozone heat the planet as much as aerosols cool it. And they don’t last nearly as long as CO2. Low-lying ozone persists a few weeks at most, and methane disappears within a decade.
So, eliminating those pollutants next to aerosols could negate any sudden warming that would otherwise occur. “But that’s opposite of what happened,” Allen said. Not only has ship-released sulfur completely disappeared, but China has put in a lot of effort clears his air at a rate faster than anyone expected. Meanwhile, carbon emissions keep swelling around the world. Yuan and many others argue that this caused a “termination shock”, leading to a sudden rise in global temperatures with additional effects that extend beyond heat. The elimination of aerosols without an accompanying decrease in greenhouse gases has the potential to wildfire activity worsens in boreal forests, an essential ocean current slows downand influence regional weather patterns in ways that are not yet understood.
All this led to seven climate advocates and researchers penning an open letter to the International Maritime Organization. They urge it to consider allowing vessels to burn dirtier fuels on the high seas, far from population centers, to “increase the global cooling benefits of sulfur or similar aerosols without causing harm to people or natural systems.” They also ask the organization to support research and testing of technologies that will enable ships to e.g. creates salt aerosols from seawater and spray them into the air to get the benefits of brighter clouds without the side effects of sulfur.
Although a spokesman for the International Maritime Organization said it welcomes reports and research from most anyone, it only considers regulations when they are raised by member states. Whether the open letter will therefore have any impact remains to be seen. Nevertheless, the dispatch represents an early rumble of what many scientists fear could become a scream engineer the planet in increasingly dramatic and deliberate ways as the climate crisis deepens.
Such calls aside, the unintended impact of IMO 2020—to say nothing of climate change itself—makes it 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 design a precise measure of targeted cooling when scientists can’t even agree on the exact impact of our chance experiment.
