Cleaner ship fuel changed clouds, but not their climate balance, says study

Credit: azores.com

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Scientists got a surprise when finding a reduction in sulphur emissions didn’t have one of the effects they expected. This puts a spanner in the works of claims by climatologists that recent shipping emission rule changes would, or did, cause extra atmospheric warming. For example, see this Talkshop post about a year ago: “Jim Hansen and colleagues have revisited the topic of aerosol-induced warming due to reduced shipping emissions (due to regulatory changes in 2020)”. A 2023 guest post at Carbon Brief by Zeke Hausfather and Piers Forster (‘Analysis: How low-sulphur shipping rules are affecting global warming’) asserted: ‘this will contribute to warming and make it even more difficult to avoid exceeding 1.5C in the coming decades’. Time to dial back at least some of this misleading climate alarm.
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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, says Eurekalert.

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.”

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.
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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.

Full news release here.
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Image credit: azores.com