Atmospheric scientist Michael Diamond and graduate student Lilli Boss published findings in Atmospheric Chemistry and Physics showing that recent fuel regulations cutting sulfur content by about 80 percent resulted in a 67 percent reduction in cloud droplet formation compared to previous fuel standards. Diamond noted, "The unexpected rerouting of global shipping gave us a unique opportunity to quantify aerosol-cloud interactions, reducing the largest source of uncertainty in global climate projections. When your 'laboratory' is the atmosphere, it's not every day you can run experiments like this one. It was an invaluable opportunity to get a more accurate picture of what's happening on Earth."
The International Maritime Organization's 2020 mandate to lower sulfur in marine fuels increased air quality, as aerosols from ship emissions-particularly sulfate-directly affect cloud brightness and Earth's energy balance. Such aerosol-cloud interactions cause clouds to form with smaller, more numerous droplets, increasing reflectivity and contributing to cooling that has masked about a third of greenhouse warming. Unlike greenhouse gases, aerosols persist for only days or weeks, which makes climate impacts difficult to analyze.
Diamond's prior work suggested that clouds in shipping corridors displayed larger and fewer droplets since the IMO 2020 rule. The scientific community continues to debate the intensity of cloudiness reduction after the rule, with estimates between 10 percent and 80 percent.
Attacks in November 2023 caused a drop in Red Sea shipping and an increase in South Atlantic routes, where low-lying clouds are strongly influenced by ship emissions. This "natural experiment" enabled researchers to track direct responses in clouds to ship emissions, something rare in atmospheric science. Satellite data showed a marked rise in nitrogen dioxide above the southeastern Atlantic, confirming greater shipping activity and allowing comparisons of cloud droplet numbers before and after the IMO regulation.
Despite twice as many ships operating in 2024, cloud droplet formation from shipping emissions was only marginally weaker than before the sulfur limits. Comparing unaffected NO2 levels with cloud droplet numbers, Diamond and Boss quantified a 67 percent drop in the ability of ship emissions to alter clouds since the regulation.
This research narrows uncertainty about how clouds respond to aerosols, an issue central to assessing Earth's energy balance. Results suggest policymakers can better weigh environmental regulations against long-term climate objectives. The study also highlights how cleaner air policies that benefit human health by reducing sulfur pollution can interact with climate processes. Tens of thousands of premature deaths have been avoided since the implementation of the regulation.
Research Report:Conflict-induced ship traffic disruptions constrain cloud sensitivity to stricter marine pollution regulations
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