Aerosol particles emitted by ships substantially influence the microphysical properties of marine clouds. In 2020, regulatory changes led to a marked reduction in sulphur emissions, resulting in measurable decreases in cloud droplet concentrations. This study exploits a unique “accidental experiment” triggered by military conflict, which caused a rerouting of shipping traffic in the Red Sea. The authors use this event to robustly quantify how cloud droplet concentrations responded to changes in aerosol emissions associated with the changes in regulations.