Scientists studied the health of 462 coral colonies near One Tree Island in the southern Great Barrier Reef off Queensland's coast during a heatwave in 2024.
The marine heatwave was the seventh mass bleaching event on the reef since 1998 and the fifth since 2016.
It brought severe conditions and rapid coral health decline not previously seen in the southern reef region.
"What we saw at One Tree Reef was catastrophic," said Michael Kingsford, Distinguished Professor of Marine Biology at James Cook University.
A marine heatwave bleached two-thirds of the coral colonies around One Tree Island in 2024. (HANDOUT/JAMES COOK UNIVERSITY)
"Rapid high mortality left no opportunity for these corals to recover, some coral even turned to rubble."
Coral bleaching is a stress response due to changes to the environment such as increased water temperatures or freshwater flooding.
Coral has single-celled algae called zooxanthellae living inside it which gives it colour and food.
But when the coral gets stressed it expels the algae and turns white.
Scientists found that the heatwave bleached two-thirds of the coral colonies by February 2024 which rose to more than three-quarters by April.
Nearly half of the colonies were dead by May, growing to 53 per cent in July.
The study revealed 31 per cent of the coral colonies remained bleached while 16 per cent recovered.
Prof Kingsford said mass bleaching events were becoming more common on the reef, occurring every two years.
The Great Barrier Reef's prospects are "poor", with climate change its biggest threat. (HANDOUT/JAMES COOK UNIVERSITY)
The dire situation sparked a renewed call for action on global warming to prevent the Australian icon from being destroyed.
"Catastrophic conditions and dire ecosystem changes are no longer a threat on a distant horizon," he said.
"It's happening now.
"Our findings reinforce the need for urgent global action, immediately, on ambitious climate and reduced emissions target."
The study was published in the Association for the Sciences of Limnology and Oceanography.
It echoed the findings of the five-yearly Great Barrier Reef Outlook Report released last year, warning the reef's prospects are "very poor".
The report said climate change was the reef's main threat, fuelling increasingly frequent marine heatwaves that bleach and kill coral and amplify other menaces including poor water quality.
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