Great Barrier Reef summer snapshot shows widespread coral bleaching

After a “worst ever” summer, coral bleaching on the Great Barrier Reef is more severe and in more places than ever before.

The Great Barrier Reef has experienced “probably the most intense and widespread bleaching event recorded to date,” the head of the reef’s aerial monitoring program has confirmed.

The latest snapshot report, released on Wednesday morning, found a range of stressors impacted on the Great Barrier Reef over what reef experts said was the “worst summer on record,” with elevated water temperatures, two tropical cyclones, extreme rainfall events in December, January, February and March, and continuing outbreaks of the Crown of Thorns Starfish.

The snapshot found:

• Evidence of bleaching on 73 per cent of reefs surveyed;

• “Very high” levels of bleaching (betweeen 61-90 per cent of coral affected) in 31 per cent of reefs surveyed;

• Evidence of “extreme” levels of bleaching (more than 90 per cent of coral affected) in 8 per cent of reefs surveyed.

The extent of bleaching on Lizard Island. Picture: CSIRO/George Roff

Researchers monitored 1080 individual reefs from the air, over 10 clear days with low wind and low wave activity, starting towards the end of summer. They flew at low altitudes of around 500 feet, enabling visibility down to between six and ten metres.

Dr Neal Cantin from the Australian Institute of Marine Science (AIMS), who led the aerial survey work, said “This is probably going to be the most intense and widespread bleaching event recorded to date”.

“This event we’re seeing high levels of bleaching in all three regions of the marine park, from Lady Elliot Island all the way to Princess Charlotte Bay,” he said.

Results of aerial surveys of the Great Barrier Reef, summer 2023/24.

In the official research paper underpinning the snapshot, Dr Cantin and his team said the corals were experiencing “the highest levels of thermal stress on record”.

The snapshot, produced by the AIMS, along with the Great Barrier Reef Marine Park Authority (the Reef Authority) and the Commonwealth Scientific and Industrial Research Organisation (CSIRO), synthesises observations made by researchers every week during the summer season.

Despite the widespread bleaching, Dr Cantin said there were still healthy corals to be found.

“The Barrier Reef is a very large ecosystem, and there are locations that are less bleached and that won’t have mortality from this event,” he said.

Dr Neal Cantin, head of the aerial survey team for the Australian Institute of Marine Science. Picture: Supplied

Great Barrier Reef Foundation managing director Anna Marsden.

The Reef Authority confirmed the reef was undergoing a mass bleaching event on March 8. It was the fifth such event recorded since 2016.

Dr Cantin has helped develop a scale from one to five to denote the severity of a bleaching event, but the summer assessment would not be made until after the results of water quality tests were known.

The last few summers on the Reef have been strikingly different. In the 2021/22 summer, bleaching was reported on 91 per cent of all reefs surveyed, but in the “benign” 2022/23 season, coral was reported to be at record levels in the northern and central parts of the Reef.

The Summer Snapshot comes just two days after the US National Oceanic and Atmospheric Administration (NOAA) confirmed tropical reefs around the world were experiencing a fourth mass bleaching event, the second in the past decade.

This aerial photo taken on April 5, 2024, shows tourists snorkelling above bleached and dead coral around Lizard Island on the Great Barrier Reef. Picture: AFP

Since early 2023, NOAA said bleaching had been detected across the Pacific, Atlantic and Indian Oceans, as well as the Caribbean, the Persian Gulf and other areas.

Great Barrier Reef Foundation manging director Anna Marsden said this week’s reports were “devastating milestones for our Great Barrier Reef and the world’s coral reefs”.

“The confirmation that the Reef has suffered its worst summer on record amidst a fourth global bleaching event underscores what the science has been telling us for some time – coral reefs are on the frontline of climate change,” she said.

This underwater photo taken on April 5, 2024, shows fish swimming near bleached and dead coral around Lizard Island. Picture: AFP

“If we do not take immediate action to reduce global emissions and implement effective conservation measures, we risk losing these irreplaceable ecosystems forever.”

While bleaching does not necessarily mean that the coral has died, research by AIMS and James Cook University earlier this year showed it had serious implications for the future health of reefs. Researchers found one strain of coral that had been affected by mass bleaching during 2020 looked as though it had recovered, but actually had a 21 per cent decrease in reproductive output.

Australian Marine Conservation Society Campaign Manager Dr Lissa Schindler said coral could bounce back after bleaching events, “but climate change is driving more frequent and severe events, meaning reefs aren’t being given the chance to recover.”

Originally published as Great Barrier Reef summer snapshot shows widespread coral bleaching