In 2020, there was an 80% reduction in greenhouse gas emissions from ships, the result of tighter restrictions imposed by the International Maritime Organization on the sulfur content of fuels.
There are those who argue that this reduction led to an increase in global temperatures, since sulfur emissions “masked” the real warming of the planet through a temporary cooling effect in the atmosphere. This is because sulfur particles reflect more solar radiation.
In a 2025 article, researchers suggested that reductions in the amount of sulfur in maritime fuel may have contributed to the global temperature peaks observed in 2023 and 2024. A similar point was raised by the Carbon Brief organization.
However, Arindam Roy, a climate specialist at the Clean Air Fund, wrote in July of last year that caution was needed, as these studies, and others, present conclusions involving “considerable uncertainties that are still actively debated in the scientific community.”
Now, a team of scientists from the University of Utah (United States of America) published this month an article in the journal Atmospheric Chemistry and Physics, in which they explain how the reduction of maritime emissions affected cloud formation over the Azores archipelago.
The researchers found that this reduction affected the internal structure of the clouds, which came to be composed of fewer water droplets, but larger in size. Furthermore, they reveal that these effects “surprisingly” did not alter the clouds’ capacity to reflect the solar radiation, an effect known as albedo.
With fewer sulfur particles in the atmosphere, there were also 15% fewer condensation nuclei—particles such as dust, smoke, ash or salts—around which water droplets or ice crystals form.
In times of high sulfur emissions, the researchers explain, the clouds had many more particles around which they could condense. Therefore, the droplets were smaller, because the water was divided among more condensation nuclei.
This investigation, led by Gerald Mace, aimed to determine whether the reduction of maritime sulfur emissions could accelerate climate change by changing the properties of the clouds that form over the Atlantic. Although some had theorized in that direction, Mace’s team says they found something different: the reduction of sulfur emissions by ships did not reduce the Atlantic clouds’ albedo, which maintained their capacity to reflect solar radiation.
Therefore, the scientists conclude that this reduction in emissions did not contribute to the warming of the planet, thus, they argue, putting an end to the debate.
“Somehow, the climate system adjusted in such a way that the radiative effect of these clouds remained balanced,” says Mace.
Based on the data they collected, the team says that changes in the region’s cloud cover will be more associated with changes in weather patterns than with changes in aerosol concentrations, “suggesting that multiple factors govern how clouds behave.”
However, these conclusions are spatially limited, as they pertain to what happens in the North Atlantic, and Mace cautions against generalizations.
“It is a regional finding and I cannot generalize it without much more data,” he says.
“What we can conclude from this study is that the climate system is capable of adjusting in ways that can be counterintuitive and that simple arguments often need to be considered more carefully.”
