The Great Barrier Reef has suffered its most significant coral decline in nearly four decades, according to a report released on Wednesday by the Australian Institute of Marine Sciences (AIMS).

The reef’s northern and southern zones recorded the steepest annual drop in hard coral cover since AIMS began monitoring the ecosystem 39 years ago. Coral loss ranged from 25% to 33%, reversing recent years of steady growth.

Mike Emslie, who leads AIMS' long-term monitoring programme, said the system is displaying signs of distress.

“We are now seeing increased volatility in the levels of hard coral cover,” he explained, adding, “This is a phenomenon that emerged over the last 15 years and points to an ecosystem under stress.”

The reef, which stretches 2,400 kilometres off Queensland's coast, is the world’s largest living structure and a critical marine habitat. It has endured five mass coral bleaching events since 2016, caused by extreme heat linked to climate change.

The 2024 bleaching episode was the most extensive ever recorded, with “high to extreme” bleaching reported across all three reef regions, AIMS said.

Despite these alarming findings, the Great Barrier Reef has not been added to UNESCO’s list of world heritage sites in danger. However, the UN body has maintained its recommendation for such a listing, citing ongoing environmental threats.

Australia, which earns A$6.4 billion ($4.2 billion) annually from reef-related tourism, has repeatedly lobbied against the designation, warning it could affect international visitor numbers.

The federal government has committed funding and introduced climate adaptation measures, but environmental groups say more aggressive action is needed to curb carbon emissions and protect the reef.

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