A recent scientific investigation has uncovered a paradoxical effect of environmental regulations in the shipping industry, revealing that efforts to reduce sulphur emissions inadvertently intensified a coral bleaching event on Australia's Great Barrier Reef. The study, led by atmospheric scientists, indicates that cleaner fuels allowed more solar energy to reach the reef, exacerbating heat stress during a critical period.
Unintended Consequences of Sulphur Reduction
In 2020, the International Maritime Organization, a United Nations agency, implemented stringent regulations to cut the sulphur content in shipping fuels from 3.5% to 0.5%. This move aimed to mitigate respiratory health issues and acid rain caused by sulphur pollution. However, sulphur aerosols also have a shading effect by brightening clouds and reflecting sunlight, which provides natural cooling to areas beneath.
Dr Robert Ryan, an atmospheric scientist at the University of Melbourne who spearheaded the research, explained that the removal of sulphur, while necessary for human health, led to "a lot of extra sunlight getting on to the reef." This increase in solar radiation contributed significantly to heat stress during the 2022 mass bleaching event.
Impact on the Great Barrier Reef
The Great Barrier Reef experienced its sixth mass bleaching event in early 2022, notable for occurring during a typically cooler La Niña year. By analysing ship data from 18 to 28 February 2022, when heat was accumulating, researchers estimated that around 5,000 vessels were in the region. Using advanced computer models, they compared scenarios with and without the sulphur regulations.
The findings showed that prevailing winds would have blown sulphur emissions over the reef, providing shade. Without these aerosols, the study reported that the reef's waters absorbed between 5% and 10% more of the sun's energy. This translated to up to 10% additional heat stress on the corals, accelerating bleaching processes.
Broader Climate Implications
Global heating, primarily driven by fossil fuel emissions, has elevated ocean temperatures and increased the frequency and severity of coral bleaching episodes. The Great Barrier Reef has suffered seven mass bleaching events since 1998, with five occurring in the past decade alone. While sulphur emissions dissipate within days, carbon dioxide remains in the atmosphere for centuries, underscoring the need for simultaneous reductions in both pollutants.
Dr Ryan emphasised that environmental policies must consider interconnected effects: "Nothing happens in isolation. There's no such thing as a free lunch." This sentiment was echoed by Prof Steven Sherwood, a climate scientist at the University of New South Wales, who noted that although the global warming impact of sulphur removal is small (up to 0.05°C), local effects in tropical regions like the reef can be more pronounced, potentially reaching 0.15°C.
Understanding Coral Bleaching
Coral bleaching occurs when corals expel the algae living in their tissues due to stress from factors like elevated water temperatures. This process leaves the coral's white skeleton visible, giving it a bleached appearance. While corals can survive mild bleaching, extreme or prolonged heatwaves can lead to mortality, reduced growth, and increased disease susceptibility. The shortening intervals between bleaching events hinder reef recovery, threatening ecosystems that support fisheries and tourism industries worldwide.
The research, published in the journal Communications Earth & Environment, highlights the complex trade-offs in environmental policymaking. It calls for integrated strategies that address both air quality and climate change to protect vulnerable marine habitats like the Great Barrier Reef from further degradation.
