ABOVE: Damaged coral near Townsville, Australia, in 2016 © Commonwealth of Australia (Great Barrier Reef Marine Park Authority) 2016, photographer: C. Jones.

More than 90 percent of the already-beleaguered Great Barrier Reef has suffered coral bleaching this year, in the fourth mass bleaching event in seven years and the sixth on record.

“This is heartbreaking. This is deeply troubling,” Simon Bradshaw, a researcher at the Australia-based Climate Council, tells the Associated Press. “It shows that our Barrier Reef really is in very serious trouble indeed.”

A delayed report published this week by the Great Barrier Reef Marine Park Authority found that 654 of 719 reefs surveyed between 12–23 March of this year had experienced some level of bleaching, in which high water temperatures trigger the corals to expel their symbiotic algae and lose their color, which puts them at risk of starvation and disease.

The severity of bleaching varied across the reef, with the worst effects seen in the shallows of the central part of the reef off the coast of Townsville, Queensland. Here, 87 percent of the overall reef area experienced major bleaching—defined as 31 percent to 60 percent of reef communities bleached—and some individual coral reefs were more than 90 percent bleached. 

The bleaching event occurred due to unusually high water temperatures beginning in December 2021, during Australia’s summer. In March 2022, water temperatures were one to three degrees Celsius higher than normal for that time of year.

The timing is unusual in that it is the first mass bleaching event to happen during a La Niña year—the cooler, wetter phase of the El Niño-Southern Oscillation in the Pacific Ocean. 

Marine biologist Terry Hughes from James Cook University in Townsville, who was involved in the reef survey, writes on Twitter that the bleaching was even more severe than that seen in 1998, which was then the hottest year on record. 

He adds that the deadliest bleaching event was the 2016 mass bleaching, which occurred during an El Niño summer and killed 30 percent of corals. “The next strong El Niño will be awful,” he writes.

This represents the sixth mass bleaching event on record to hit the Great Barrier Reef, all of which have occurred in the past two decades. The Great Barrier Reef Marine Park Authority notes in its report that climate change is the greatest threat to the reef. “Only the strongest and fastest possible actions to decrease global greenhouse gas emissions will reduce the risks and limit the impacts of climate change on the Reef.”

See “Environmental Memory: How Corals Are Adjusting to Warmer Waters

The news comes as Australians prepare to vote on May 21 in a federal election that has been dominated by issues around climate change and against a backdrop of what the independent international Climate Tracker initiative ranks as “highly insufficient” action on climate change from the incumbent conservative coalition. 

The report on the health of the Great Barrier Reef was expected to be released earlier in the year, after surveys were conducted in March. Hughes tells The Guardian that the maps were usually released within ten days of the surveys, and previous reports on mass bleachings were published within two weeks.

A spokesperson for the Great Barrier Reef Marine Park Authority told the Sydney Morning Herald newspaper in early May that the report would likely be delayed until after the election. The survey findings were then released without governmental fanfare on May 10.