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

Climate change is increasing the frequency of mass coral bleaching events at Australia’s Great Barrier Reef (GBR), but its sensitivity to changing atmospheric aerosol levels is not well understood. Global shipping fuel sulfur content regulations introduced in 2020 (“FSC05”) significantly reduced sulfate aerosol levels over the oceans. This has improved air quality in port cities but reduced the direct and indirect radiative cooling effects of sulfate aerosol, including at the GBR. We use the WRF-Chem model to simulate ship emission impacts on aerosols, clouds and solar radiation for the GBR during February 2022, in the lead-up to that year’s unprecedented La Nina mass coral bleaching event. Ship emissions reduce incoming shortwave radiation (SWR) by 2.3 Wm−2, compared to a scenario with no ships, but prior to FSC05 regulation the SWR reduction would have been much larger at 11 Wm−2. Relative to pre-2020 conditions, the 80 % reduction in sulfate aerosol produces a de-masking of climate change impacts at the GBR. This is likely equivalent to an additional 0.25◦C of sea surface temperature heating, or up to 3 additional degree heating weeks of coral thermal stress, acting on top of the global surface warming attributable to reduced shipping sulfate.