REGULATIONS governing ship fuel sulphur content may be contributing to mass coral bleaching events on the Great Barrier Reef (GBR).
A paper led by atmospheric scientist, Dr Robert Ryan from the University of Melbourne, published 22 January, said global shipping fuel sulphur content regulations introduced in 2020 have reduced the radiative cooling effects of sulfate aerosol over the ocean.
The paper says that while the removal of sulphur from fuels was necessary, an unforeseen side effect was allowing a lot of extra sunlight onto the reef.
The paper says climate change is increasing the frequency of mass coral bleaching events on the Great Barrier Reef, and clear skies are allowing more radiation to reach the reef.
During February 2022, the build-up to a La Niña mass coral bleaching event, the scientists measured increased shortwave radiation reaching the reef.
The post-sulfate regulation results, compared with the control pre-regulation scenario, showed dominant clear skies with fewer cloudier and windier periods.
It said this was likely to 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 Great Barrier Reef corals now, compared with pre-regulation of ship sulfate emissions.
The paper said the reef is under severe threat from climate change.
“As ocean temperatures increase, coral bleaching events are becoming increasingly frequent, with mass bleaching events occurring in 1998, 2002, 2016, 2017, 2020, 2022 and 2024,” the paper said.
“Coral bleaching occurs as the symbiotic relationship between corals and their algal symbiont organisms breaks down under thermal stress, leaving the coral exposed and at high mortality risk.
“While rising ocean temperatures are understood to be the primary cause of mass coral bleaching, enhanced solar radiation has also been associated with GBR bleaching events and research suggests the severity of bleaching events can be mitigated by natural or artificial shade.
“The opportunity of shading reefs has led scientists to propose marine cloud brightening (MCB) on the GBR, which utilises aerosols and aerosol–cloud interactions to reduce the amount of solar radiation reaching the ocean surface.
“Simultaneously, an unintentional geoengineering effect has occurred over the world’s oceans, including the GBR, with ship aerosol and gaseous emissions perturbing the local radiation balance.
“Primary and secondary sulphate aerosol from ships interacts with radiation directly, as well as indirectly by acting as cloud condensation nuclei (CCN).
“Extra CCN alters cloud properties by dividing the available cloud water among more droplets, decreasing the size of the cloud droplets and thereby enhancing cloud albedo (a measure of how much sunlight clouds reflect back into space), cloud lifetime, and optical depth.
“Ship-polluted clouds can also exhibit liquid water path increases, which also enhances cloud albedo.
“This effect has been observed in satellite images of ship tracks, with marine cloud cover and albedo enhanced above regions of high ship traffic.
“Recent research also shows a substantial radiative forcing effect due to liquid water path enhancements within marine clouds, even when ship tracks are not visible in satellite images.”
The paper said Australia relies on the maritime industry for more than 99% of its imports and exports by volume, and shipping is expected to constitute a large portion of the country’s transport sector into the future.
Ships in the GBR region cause a deterioration in air quality in and around Queensland port cities and coastal regions as well as other parts of Australia, Europe, Asia, and the Americas.
“Primary emissions from the shipping industry include sulphur dioxide (SO2), carbon monoxide (CO) as well as sulfate (), black (BC) and organic carbon (OC) aerosol,” the report said.
“Ships also emit greenhouse gases such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) as well as perturbing tropospheric ozone (O3) and CH4 by emission of volatile organic compounds (VOCs) and nitrogen oxides.”