A new study indicates that the removal of sulphur from shipping fuels intensified a significant coral bleaching event on the Great Barrier Reef by allowing more solar energy to reach the reef.
Sulphur pollution is associated with respiratory issues in humans and contributes to acid rain. It also has a shading effect, capable of brightening clouds and providing shade.
Dr. Robert Ryan, an atmospheric scientist at the University of Melbourne and lead researcher, stated that the removal of sulphur from fuels, which he considers necessary, paradoxically resulted in "a lot of extra sunlight getting on to the reef."
The study reported that corals on the Great Barrier Reef experienced up to 10% more heat stress during a 2022 mass bleaching event. This was attributed to reduced sulphur emissions from thousands of ships in the region.
In 2020, the UN's International Maritime Organization implemented regulations reducing the permissible sulphur content in fuels from 3.5% to 0.5%.
The Great Barrier Reef experienced its sixth mass bleaching event in early 2022. This was notable as it occurred during a La Niña year, which typically brings cooler conditions.
Researchers, including Ryan, analyzed ship data from the region between February 18 and 28, 2022, a period when heat was accumulating on the reef. Approximately 5,000 ships were in the area. Computer models were used to assess sulphur emissions under scenarios with and without the new regulations, and to determine the atmospheric movement of sulphur based on weather conditions.
Ryan stated that the study found between 5% and 10% more solar energy reached the reef's waters, compared to pre-regulation conditions. This was due to prevailing winds carrying sulphur over the reef.
Coral Bleaching Explained
Coral bleaching occurs when corals expel the symbiotic algae that provide their color and nutrients, exposing their white skeletons. This phenomenon is largely driven by rising ocean temperatures, although sunlight also plays a role. While corals can recover from bleaching under mild conditions, extreme marine heatwaves can be fatal. Bleaching also increases susceptibility to disease and impairs growth and reproduction. Scientists observe that recovery periods for reefs are shortening due to increasingly frequent bleaching events. Coral reefs are recognized as highly vulnerable ecosystems, essential for supporting fisheries and tourism globally.
The Great Barrier Reef, the world's largest coral reef system, has experienced seven mass bleaching events since 1998, with five of these occurring in the last decade.
Global heating, primarily attributed to fossil fuels, has contributed to rising ocean temperatures, leading to more frequent and severe coral bleaching on the reef.
Widespread mass bleaching on the reef was observed in 1998, 2002, 2016, 2017, 2020, 2022, and 2024.
Ryan noted that CO2 emissions persist in the atmosphere for over a century, while sulphur emissions dissipate within days. He suggested that both air pollutants and greenhouse gases require simultaneous reduction, emphasizing that environmental actions have interconnected effects.
Professor Steven Sherwood, a climate scientist not involved in the study, stated that prior research indicates sulphur aerosol removal from shipping has resulted in a global temperature increase of up to 0.05C. Sherwood commented that while the global climatic impact is minor, this study suggests a more significant local effect on tropical ocean temperatures. This is attributed to air pollution absorbing sunlight in addition to reflecting it. Sherwood indicated that the study identified a larger local effect, potentially up to 0.15C, which is sufficient to increase heat stress on corals. He described this as an important finding, showing a non-trivial impact on corals despite a small overall global warming effect.
The research findings were published in the journal Communications Earth & Environment.