Efforts to slash air pollution from the global shipping industry have delivered a major public health win by curbing sulphur emissions — but a new study reveals an unexpected downside: the cleaner skies inadvertently intensified a mass coral bleaching event on Australia's Great Barrier Reef.
The research, published in Communications Earth & Environment, examines the impacts of the International Maritime Organization's (IMO) 2020 regulation, which cut the maximum sulphur content in ship fuels from 3.5% to 0.5%. Known as IMO 2020 or the "FSC05" rule, the measure dramatically reduced sulphate aerosol emissions from vessels. These tiny particles act like a planetary parasol, reflecting sunlight back into space and exerting a cooling influence on the Earth's surface — a phenomenon sometimes called "global dimming" from ship tracks.
With fewer aerosols in the atmosphere after the rule took effect, more solar radiation reached the ocean surface. During February 2022 — the build-up to a severe bleaching episode on the reef — the study found an additional 11 watts per square metre of daytime downward shortwave radiation in shipping-heavy corridors around the Great Barrier Reef. This translated to up to 10% more heat stress on corals compared with what would have occurred under pre-2020 emission levels.
The 2022 event was already remarkable: it marked the sixth mass bleaching on the reef since 2016, and the first to unfold during a La Niña phase, which typically brings cooler, cloudier conditions to the region and offers some natural protection against marine heatwaves. Despite those favourable weather patterns, widespread bleaching occurred, affecting large swathes of the northern and central sections.
Lead author Dr Robert Ryan, an atmospheric scientist at the University of Melbourne, described the findings as highlighting a classic trade-off in environmental policy. "Reducing sulphur in fuels was absolutely the right thing to do for human health — sulphur dioxide contributes to acid rain, respiratory problems and premature deaths," he said. "But the aerosols also masked some of the warming from greenhouse gases. Removing them is like lifting a shade: more sunlight gets through, and in sensitive systems like coral reefs, that extra energy can tip the balance."
The study used the WRF-Chem atmospheric model to simulate ship emission scenarios over the reef during the critical February period in 2022. It compared post-IMO 2020 conditions with hypothetical pre-regulation levels and a no-ships baseline. Results showed that shipping emissions, even after the sulphur cap, still provided some cooling (about 2.3 W/m² reduction in incoming radiation compared with no ships at all). But the drop from pre-2020 levels equated to a "de-masking" effect — roughly equivalent to an additional 0.25°C of sea surface warming locally, or up to three extra degree-heating weeks of thermal stress piled on top of baseline climate change pressures.
Degree-heating weeks (DHW) measure cumulative heat exposure; values above 4 often trigger severe bleaching, while 8 or more can cause widespread mortality.
The irony is stark. The shipping lanes threading past the Great Barrier Reef carry thousands of vessels annually, making the region one of the world's busiest for aerosol-producing traffic. Cleaner fuels mean healthier air for coastal communities and reduced acid rain, but in this case, they amplified ocean heating precisely when corals were most vulnerable.
Experts stress that the IMO rule's benefits far outweigh the localised drawbacks — global sulphur cuts prevent tens of thousands of premature deaths each year from air pollution. Yet the findings underscore the need for more integrated approaches to climate and pollution policy.
"Environmental fixes don't happen in isolation," said a spokesperson for the Great Barrier Reef Marine Park Authority. "We already face escalating marine heatwaves from greenhouse gas emissions. This study shows how even positive regulations can have rebound effects in fragile ecosystems."
The reef has shown signs of recovery in some areas since 2022, aided by relatively mild conditions in intervening years, but scientists warn that consecutive or intensified bleaching events — now occurring against a backdrop of rapidly rising baseline temperatures — threaten its long-term survival.
As the world grapples with balancing immediate pollution controls against accelerating climate impacts, the Great Barrier Reef offers a sobering reminder: in complex Earth systems, solving one problem can quietly exacerbate another.