Telltale for the New IMO Ship’s Fuel Regulations Causing Significant Warming Since 2023: Atmospheric Nitrogen Oxide Emissions
There has been a 67% reduction in ships’ cloud-altering abilities after the International Maritime Organization’s ship’s fuel regulations limiting sulfur went into effect. Sulfur is a natural component of fossil fuels that when burned creates air pollution responsible for 7 to 8 million deaths per year globally. Burning sulfur in fossil fuels creates sulfur oxides that are global cooling pollutants that counter global warming. These global cooling pollutants are called “bright aerosols” and they reflect sunlight harmlessly back into space without the light striking an object and changing into heat that is trapped in our atmosphere by the greenhouse effect. They not only directly reflect light back into space, but they indirectly do so too, by directly causing ship exhaust condensation trails, and by causing clouds to be whiter and more reflective. Aerosols, mostly sulfur oxides, have masked 46 percent of human-caused CO2 warming and about a third of all human-caused warming.
(Abstract, Diamond and Boss 2025) Starting in November 2023, the Houthi militia occupying northwestern Yemen has attacked ships passing through the Bab al-Mandab Strait, a chokepoint on the Europe-Asia route via the Suez Canal. Cargo ship traffic through the Red Sea has since plummeted, with ships instead taking the longer route around the Cape of Good Hope. The increase in traffic in the southeastern Atlantic Ocean is readily apparent in satellite retrievals of nitrogen dioxide. Within the stratocumulus deck covering much of the southeastern Atlantic, a previously detectable cloud microphysical perturbation due to ship pollution had largely disappeared following the International Maritime Organization’s sulfur-limiting regulations in 2020 but returns during 2024 due to the increase in ship traffic despite the lower cloud brightening efficacy per ship. Because nitrogen dioxide pollution per unit of fuel oil burned is not affected by switching to low-sulfur fuel, quantifying the ratio of shipping enhanced cloud droplet number and nitrogen dioxide concentrations before and after the fuel sulfur limits went into effect provides a constraint on the cloud changes from the regulations. We find that the ∼ 80 % reduction in sulfur emissions leads to a ∼ 67 % reduction in the increase in cloud droplet number concentration per unit marine fuel oil burned.)
Diamond and Boss, Conflict-induced ship traffic disruptions constrain cloud sensitivity to stricter marine pollution regulations, Atmospheric Chemistry and Physics, November 21, 2025.
https://acp.copernicus.org/articles/25/16401/2025/acp-25-16401-2025.pdf