New research suggests that global shipping regulations intended to improve air quality may have inadvertently caused a climate reaction that is warming the planet.
In January 2020, the International Maritime Organization reduced the amount of sulfur dioxide (SO2) content allowed in shipping fuel from 3.5% to 0.5%. This shift resulted in an estimated 80% reduction in harmful SO2 emissions from ships worldwide, as the Los Angeles Times detailed.
The hope was to help reduce adverse health impacts on those who lived near ports — such as higher risks of stroke, asthma, lung cancer, and other diseases tied to sulfur dioxide and related pollution. Researchers have estimated that the regulations could avoid 30,000 premature deaths each year, according to MIT.
The regulations may have caused an unexpected consequence, according to a new study published in Communications Earth & Environment. Sulfur dioxide emissions, though inarguably harmful to public health, spurred the formation of bright clouds over the ocean. These clouds reflected some of the sun's light and energy back into space, helping to deflect otherwise warming rays from reaching Earth. Live Science describes the effect as being like a "giant planetary sunblock."
With sulfur dioxides in shipping fuel reduced, fewer bright clouds formed, allowing more of the sun's heat and energy to reach the Earth's surface. The new report says that this increase in energy partly contributed to high global temperatures in 2023 — the planet's hottest year on record.
The study estimates that the change could lead to a global temperature increase of 0.16 of a degree Celsius (about 0.29 of a degree Fahrenheit) within the next seven years. That "could lead to a doubling (or more) of the warming rate in the 2020s compared with the rate since 1980," according to the study.
While this may all sound alarming, not every climate scientist agrees with the findings. Zeke Hausfather, a climate scientist with Berkeley Earth, told the Los Angeles Times that there is a broad consensus that the change in shipping regulations contributed to some planetary warming — but the magnitude of that change is not agreed upon.
"They used a very simple model," Hausfather told the Times of the recent study. "And in the real world, we know from more complex climate models and from historical relationships between [climate] forcing and warming, that there's bigger lags in the system, and that not as much warming is going to be realized as quickly as they assumed."
The Times also notes that at least three other upcoming studies on the regulation change found different results. Notably, these studies vary in the degrees of warming they contribute to the new shipping regulations, ranging from 0.03 to 0.2 degree Celsius of warming.
"I think everyone is pointed in the right direction here," Hausfather added. "This had an effect — it was not a negligible effect — but exactly how big it is is something we still need to work out as a community."
Regardless of the impact, no climate scientist will advocate adding sulfur dioxide back into shipping fuel any time soon. This increase in warming, though bizarre and unintended, is not an indication that sulfur dioxide is a good thing for the climate — or public health.
"Air quality is a much bigger killer, and in a much more uneven way, than a small contribution to warming, and so it makes perfect sense to improve air quality," Daniele Visioni, an assistant professor of Earth and Atmospheric Sciences at Cornell University, told the Times. "These kind of studies should in no way be seen as a way to say we should stop improving our air. On the other hand, it does highlight the fact that the climate system is complex, and that no policy is perfect."
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