To reduce air pollution associated with ocean transport, the International Maritime Organization tightened restrictions on sulfur content in ship fuel, resulting in an 80% reduction in emissions by 2020. That shift created an inadvertent real-world experiment in how man-made aerosols influence cloud formation over the ocean.

A team of atmospheric scientists led by University of Utah professor Gerald “Jay” Mace used this rare opportunity to explore the impact of reduced emissions on marine boundary layer clouds over the eastern North Atlantic. They discovered clouds’ internal structure changed, featuring fewer, but larger droplets of water. Yet the clouds’ reflectivity of sunlight surprisingly remained unchanged.

“You couldn’t plan this type of thing,” Mace said. “The shipping in the entire world went from one thing to another, almost like the flick of a switch, and it just so happened that that had a known effect on clouds globally. Doing a natural experiment like this, I don’t think it could ever happen again, unless we went back to sulfur fuels.”

Fewer aerosols result in fewer water droplets forming

With fewer sulfur particles in the air, there were approximately 15% fewer cloud condensation nuclei, the tiny particles on which cloud droplets form, according to the findings reported in the journal ACP Letters.

“There’s a certain amount of water available to condense, and it condenses on whatever the local aerosols are,” Mace said. In the more polluted clouds in the higher sulfur fuel, “the clouds in the ship tracks then had a lot more aerosol to condense onto. So that available water had been divvied up into a higher number of droplets that are smaller.”

Now the clouds’ microphysics has been altered thanks to the sudden reduction in sulfur particles, a finding that Mace expected and that others had found.

Mace’s team examined observational data collected around the Azores, the archipelago in the Atlantic nearly 900 miles west of Portugal, for two-year periods immediately before and after the fuel regulations took effect in 2020. This study area, situated amidst busy shipping lanes between Europe and North America, was selected because it is the location of the Atmospheric Radiation Measurement program’s (ARM) Eastern North Atlantic site, operated by the U.S. Department of Energy.

The cloud cover changed, but its reflectivity remained unchanged

Mace initiated this research project to determine whether the reduction in shipping emissions, resulting from the new fuel requirements, would accelerate climate change by changing cloud properties over the Atlantic. Some scientists theorized that might happen, but Mace’s study found that not to be the case.

It turned out that the clouds after the fuel change held more total water overall, which balanced out changes in droplet size and maintained cloud solar reflectivity, known as albedo. Had the albedo of Atlantic clouds reduced, they would have reflected less of the sun’s heat back into space, thereby warming the climate.

These counterintuitive findings prompted his team to look at observations recorded by two of NASA’s orbiting Earth-observing instruments, MODIS and CERES.

The satellite imagery confirmed there was little to no change in cloud reflectivity and optical depth in the region around the Azores.

“When we looked at the water paths, they had shifted just enough, just in the right way to completely offset the radiative effect of this change,” Mace said. “Somehow the climate system had adjusted itself so that the radiative effect of these clouds maintained balance.”

Mace noted that precipitation did increase slightly, as expected with the increase in droplet size, but that increase came in the form of light drizzle. There was an actual decrease in heavy rainfall, enough to explain the change in the liquid water path the team observed.

So it is likely that changes in the amount of cloud cover over the region were more closely linked to shifts in weather patterns than to aerosol changes, suggesting multiple factors govern how clouds behave.

Mace is hesitant to suggest his findings apply globally.

“It’s a regional finding, and I can’t generalize it without a lot more data,” he said. “What we can conclude from this study is that the climate system is able to adjust itself in ways that can be counterintuitive and simple arguments often need to be considered more carefully.”

The study appeared Jan. 22 under the title, “Impact on Cloud Properties of Reduced-Sulphur Shipping Fuel in the Eastern North Atlantic,” in the journal ACP Letters, published by the European Geosciences Union. Co-authors include Sally Benson, Peter Gombert and Tiffany Smallwood of the University of Utah’s Department of Atmospheric Sciences. Funding came from the National Science Foundation, NASA and the U.S. Department of Energy.