Parts of the Great Barrier Reef have suffered the steepest annual loss of coral cover since records began almost 40 years ago, according to a new report.

Both the northern and southern stretches of the vast Australian reef experienced their most extensive coral bleaching to date, the Australian Institute of Marine Science (AIMS) has found.

Although recent tropical cyclones and outbreaks of crown-of-thorns starfish – which feed on coral – have caused further damage, AIMS said climate change-driven heat stress remains the primary factor.

The institute has warned that the reef may be nearing a tipping point, where coral can no longer recover quickly enough between catastrophic events, leaving it facing an increasingly “volatile” future.

Between August 2024 and May 2025, AIMS surveyed the health of 124 reefs – part of a monitoring programme it has conducted since 1986.

Often described as the world’s largest living structure, the Great Barrier Reef stretches for 2,300km (1,429 miles) and supports extraordinary biodiversity. But repeated bleaching events are transforming vast areas of vibrant coral into ghostly white.

Australia’s second-largest reef, Ningaloo on the west coast, has also endured repeated bleaching. This year marked the first time both major reefs simultaneously turned white.

Coral, sometimes called the “architect of the sea”, forms the foundations of ecosystems that support around a quarter of all marine species.

Bleaching occurs when coral becomes stressed in water that is too warm. A rise of 1°C above its thermal threshold for two months is usually fatal, while a 2°C rise can kill within a month.

Exceptionally warm tropical waters triggered mass bleaching on the Great Barrier Reef throughout 2024 and early 2025 – the sixth such event since 2016. Natural climate patterns such as El Niño can intensify these episodes, but climate change remains the driving force.

The reef has “experienced unprecedented levels of heat stress, which caused the most spatially extensive and severe bleaching recorded to date,” the report found.

Recovery, the report notes, could take years and will depend on successful coral reproduction combined with minimal environmental disturbance.

Among the most severely affected species are Acropora corals, which are highly vulnerable to heat stress and heavily preyed upon by crown-of-thorns starfish.

“These corals are the fastest to grow and are the first to go,” AIMS research lead Dr Mike Emslie told ABC News.

“The Great Barrier Reef is such a beautiful, iconic place, it’s really, really worth fighting for. And if we can give it a chance, it’s shown an inherent ability to recover,” he said.

The Australian government’s culling programme has destroyed more than 50,000 starfish using injections of vinegar or ox bile. While the creatures are native to the reef, their numbers have soared since the 1960s, most likely fuelled by nutrient run-off from agriculture.

“Due to crown-of-thorns starfish control activities, there were no potential, established, or severe outbreaks of crown-of-thorns starfish recorded on Central GBR reefs in 2025,” the AIMS report noted.

Richard Leck of WWF described the reef as “an ecosystem under incredible stress”, warning that scientists fear the time will come when it no longer recovers as it has in the past. He added that some coral reefs worldwide are already beyond recovery – and the Great Barrier Reef could share the same fate without urgent and ambitious climate action.

The reef has held World Heritage status for more than four decades, but Unesco has cautioned that the Australian landmark is “in danger” from warming seas and pollution.

 

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