| 12 Aug 2025
Conflict-induced ship traffic disruptions constrain cloud sensitivity to stricter marine pollution regulations
Abstract. Starting in November 2023, the Houthi militia occupying northeastern 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 detectible 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 ~66 % reduction in the increase in cloud droplet number concentration per unit marine fuel oil burned.
The paper uses the change in cloud drop number concentration ratioed to the change in NO2 to assess how the IMO’s implementation of sulfur-limiting regulations in 2020 affected cloud brightening due to ship tracks. The SE Atlantic is the area of focus due to recent increases in ship traffic as ships transit around the Cape of Good Hope rather than through the Suez Canal to avoid Houthi militia attacks. Comparing years 2019 and 2024, a large decrease in the ability of ship emissions to impact Nd is found. The paper is concise and well-written. The figures are of high quality and effectively display the results reported in the paper. I only have a few minor comments.
Line 35: “the estimated magnitude of the IMO 2020 effect ranges from ~15-75%”. Is this based on the reduction in detectible ship tracks? Please clarify.
Lines 67 – 68: “Because the 2018 NO2 values have a particularly low tail and 𝑁d values have a high tail…” I could be interpreting this statement incorrectly but the 2018 high tail of the Nd values is not evident in Figure 2b.
Line 88: Does the “binary detection threshold metric” refer to relying on only two years to assess the cloud response to the decrease in ship sulfur emissions?