A recent study has revealed that over 40% of individual corals monitored around One Tree Island in the Great Barrier Reef perished last year due to an extensive coral bleaching event – the most widespread ever recorded in the reef system.

Scientists observed 462 coral colonies after heat stress in early 2024 caused them to turn white. The researchers described the situation as “catastrophic,” with only 92 colonies remaining unbleached. By July, when the study concluded, 193 colonies had died, while 113 still exhibited bleaching effects.

Leading the study was Prof. Maria Byrne, a marine biologist at the University of Sydney, who has been studying and visiting One Tree Island for 35 years. She noted that the extent of bleaching was the worst ever seen at the reef.

“Seeing those really massive colonies die was really devastating,” she said. “I have gone from being really sad to being really cranky. We have been trying to get the message across about climate change for ages.”

The Australian Institute of Marine Science (AIMS) conducted a separate assessment in November, surveying eight reefs within the Capricorn-Bunker sector. Their findings indicated the largest annual drop in hard coral cover recorded in the region since monitoring began in the mid-1980s, with a decline of 41%. AIMS scientists also observed similar coral cover losses in the northern sections of the reef, where one government scientist described witnessing a “graveyard of corals.”

Byrne and her team initiated their study in February 2024, employing temperature loggers, video documentation, and direct observations to assess the health of 12 coral types.

The scientists wrote in the study: “As corals can recover from mild bleaching when water cools, there is a perception that while bleaching is bad, it is not necessarily catastrophic. What we observed at [One Tree Reef] was by contrast, catastrophic.”

Among these, Goniopora—a long-lived genus forming large boulders covered in vibrant flower-like polyps—was particularly affected. Many colonies bleached and later succumbed to black band disease, a flesh-eating infection.

Byrne warned that while some of the still-bleached corals might recover, others could ultimately die.

Co-author Dr. Shawna Foo, a coral reef scientist at the University of Sydney, has spent years studying the island’s reefs. However, after five months of tracking coral conditions, she found many colonies nearly unrecognisable, covered in algae, dead, or crumbling.

“It’s horrible to see this happen to somewhere I know really well but we were expecting this to happen because we have seen it in other parts of the reef, and other parts of the world,” she said.

March typically marks the peak of heat stress in the Great Barrier Reef. The Great Barrier Reef Marine Park Authority reported last week that ocean temperatures across much of the marine park are currently up to 1.2°C above average.

Meanwhile, the U.S. government’s Coral Reef Watch program predicts that parts of the reef north of Cooktown will experience severe heat stress, potentially leading to further widespread bleaching by mid-February. These findings underscore the increasing threats posed by climate change to the world’s largest coral reef system.

Richard Leck, the head of oceans at WWF-Australia, said: “We are yet to see the full data about last summer’s coral bleaching, but it’s clear there has been major mortality in areas from the north and this new research shows major mortality in the south.

“The reef is under more heat stress this summer, especially in the north, and there’s a risk we could see another back-to-back bleaching event. It’s a case of Russian roulette whether that occurs or not.

“We know the reef is under increasing pressure from climate change and its world heritage status is under increasing pressure.”