THE GREAT BARRIER Reef has experienced its greatest annual loss of live coral across most of its expanse in four decades of record-keeping.

An annual coral survey by the Australian Institute of Marine Science said that the total amount of coral has not changed significantly because of the growth of new coral cover, but the volume of coral lost is unprecedented and concerning.

The change underscores a new level of volatility on the Unesco World Heritage Site, the report said.

Mike Emslie, who heads the tropical marine research agency’s long-term monitoring programme, said that “these are substantial impacts and evidence that the increasing frequency of coral bleaching is really starting to have detrimental effects on the Great Barrier Reef”.

“While there’s still a lot of coral cover out there, these are record declines that we have seen in any one year of monitoring,” he added.

Emslie’s agency divides the Great Barrier Reef, which extends 1,500 kilometres (900 miles) along the Queensland state coast, into three similarly sized regions: northern, central and southern.

Living coral cover shrank by almost a third in the south in a year, a quarter in the north and by 14% in the central region, the report said.

Global coral bleaching

Because of record global heat in 2023 and 2024, the world is still going through its biggest – and fourth ever recorded – mass coral bleaching event on record.

Heat stress is hurting nearly 84% of the world’s coral reef area, including the Great Barrier Reef, according to the US National Oceanic and Atmospheric Administration’s coral reef watch.

So far, at least 83 countries have been impacted.

This bleaching event started in January 2023 and was declared a global crisis in April 2024.

It has eclipsed the previous biggest global coral bleaching event, from 2014 to 2017, when 68.2% of coral reefs had bleaching from heat stress.

An area diverse coral assemblage despite impacts from the 2024 bleaching event at Pompey Reef at the northern Great Barrier Reef in far North Queensland Australian Institute of Marine Science / PA Australian Institute of Marine Science / PA / PA

Large areas around Australia, though not the Great Barrier Reef, hit the maximum or near maximum of bleaching alert status during this latest event.

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In March, Australia started aerial surveys of 281 reefs across the Torres Strait and the entire northern Great Barrier Reef and found widespread coral bleaching.

Of the 281 reefs, 78 were more than 30% bleached.

Coral has a hard time thriving and at times even surviving in prolonged hot water.

It can survive short bursts, but once certain thresholds of weeks and high temperatures are passed, the coral is bleached, which means it turns white because it expels the algae that live in the tissue and give the corals their colours.

Bleached corals are not dead but they are weaker and more vulnerable to disease. Coral reefs can somewhat bounce back from these mass global bleaching events but often they are not as strong as they were before.

Coral reefs are considered a “unique and threatened system” due to climate change and are especially vulnerable to global warming beyond 1.5 degrees Celsius since pre-industrial times, the United Nations Intergovernmental Panel on Climate Change proclaimed in 2018.

The world has now warmed around 1.2 degrees Celsius since pre-industrial times and is expected to keep heating up if countries continue to fail to act quickly enough to curb climate change.