New research shows that steps to clean ship fuel by cutting sulphur increased coral bleaching on the Great Barrier Reef. Sulphur pollution is harmful to humans and the environment, so rules introduced in 2020 limited sulphur in ship fuels from 3.5% to 0.5%. But sulphur also helped shade the reef by blocking some sunlight. Dr Robert Ryan from the University of Melbourne led the study. He said that less sulphur meant “a lot of extra sunlight getting on to the reef.” During the 2022 mass coral bleaching event, the reef faced up to 10% more heat because about 5,000 ships nearby emitted less sulphur. This bleaching was unusual since it happened during a cooler La Niña year. Using ship data and weather models, researchers showed that prevailing winds carried sulphur over the reef, reducing sunlight. With less sulphur, the reef waters absorbed more sun energy. Climate change is already heating oceans, causing frequent coral bleaching on the Great Barrier Reef since 1998. Dr Ryan emphasized, “Nothing happens in isolation. There’s no such thing as a free lunch.” He warned that both greenhouse gases and air pollutants should be cut together. Prof Steven Sherwood of the University of New South Wales, not involved in the study, said that sulphur cuts raised local ocean temperatures by up to 0.15°C. This small rise “is enough to increase the heat stress on the corals.” The findings appear in the journal Communications Earth & Environment. In short, cleaning up ship fuels helped people but also allowed more sun to scorch the reef, showing the complex effects of pollution on the environment.