Ship fuel sulfur content regulations may exacerbate mass coral bleaching events on the Great Barrier Reef

Global shipping fuel sulphur content regulations introduced in 2020 reduced the radiative cooling effects of sulfate aerosol over the ocean. Here we use the WRF-Chem model to estimate the effect these regulations have had on aerosols, clouds and solar radiation at the Great Barrier Reef, where climate change is increasing the frequency of mass coral bleaching events. During February 2022, the build-up to a La Niña mass coral bleaching event, we find 11 Wm−2 extra daytime downwards shortwave radiation reaches the reef post-sulfate regulation, compared to the control pre-regulation scenario. The enhancement is dominated by clear-sky-only forcing changes and less severe in cloudier and windier periods. Persistent incoming shortwave radiation enhancements on the order of 5-11 Wm−2 likely lead to sea-surface temperature increases of 0.05-0.15 °C, implying that during bleaching-conducive conditions, 5-10% additional thermal stress is felt by GBR corals now than before the regulation of ship sulfate emissions. Reduced sulfate aerosols due to ship fuel regulation may increase shortwave radiation on the Great Barrier Reef, exacerbating the impact of marine heatwaves on coral bleaching, according to model analysis of ship emission impacts on aerosols, clouds and solar radiation.