This is the most widespread and most severe mass bleaching and mortality event ever recorded on the Great Barrier Reef.

Marine researcher ‘devastated’ by widespread event that is affecting coral species usually resistant to bleaching

Sharlotte Thou and Adam MortonThu 11 Apr 2024 05.53 BSTShare76

Concern that the Great Barrier Reef may be suffering the most severe mass coral bleaching event on record has escalated after a conservation group released footage showing damage up to 18 metres below the surface.

Dr Selina Ward, a marine biologist and former academic director of the University of Queensland’s Heron Island Research Station, said it was the worst bleaching she had seen in 30 years working on the reef, and that some coral was starting to die.

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The Great Barrier Reef Marine Park Authority last week said aerial surveys of more than 1,000 individual reefs revealed more than half were rated as having high or very high levels of bleaching, and a smaller number in the south – less than 10% of the total – had extreme bleaching. Only about a quarter were relatively unaffected.

It confirmed the 2,300-kilometre reef system was experiencing its fifth mass bleaching event in eight years. The authority said sea surface temperatures had been between 0.5C and 1.5C hotter than expected for this time of year.

A turtle beneath a bleached boulder coral on the Great Barrier Reef. Photograph: Grumpy Turtle Films

The Australian Marine Conservation Society on Thursday released video and photos that it said showed bleaching on the southern part of the reef extended to greater depths than had been previously reported this year.

Ward said the impact of bleaching had been extensive across 16 sites that she visited in the reef’s southern section, affecting coral species that had usually been resistant to bleaching. Some coral had started to die, a process that usually takes weeks or months after bleaching occurs.

“I feel devastated,” she said. “I’ve been working on the reef since 1992 but this [event], I’m really struggling with.”

Ward said sea temperatures at two of the sites she visited were the same at the surface and 20 metres below the surface. This was “very unusual”, and reinforced the need for rapid action to reduce greenhouse gas emissions, she said.

“What are we doing to stop the reef from being lost?” Ward said. “We cannot expect to save the Great Barrier Reef and be opening new fossil fuel developments. It’s time to act and there are no more excuses.”

Coral bleaching occurs when the coral becomes heat stressed and ejects the tiny marine algae, known as zooxanthellae, that live in its tissue and give most of its colour and energy. With the zooxanthellae gone, the coral starves and its bone-white calcium skeleton becomes visible.

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Diverse coral species including a brain coral with bleaching. Photograph: Grumpy Turtle Films

If the elevated temperature doesn’t last long, the coral can recover. Otherwise, it starts to die. In the most severe cases, the bleaching is skipped and the coral dies almost immediately, usually turning a dirty brown.

Terry Hughes, an emeritus professor at James Cook University and longtime reef bleaching researcher, said the aerial surveys showed “the most widespread and most severe mass bleaching and mortality event ever recorded on the Great Barrier Reef”.

He said the scale of the damage was comparable to 2016, the worst previous year experienced, but there were now fewer individual reefs untouched by bleaching between southern Queensland and the Torres Strait. He said the area south of Townsville had been particularly badly hit this year.

“We’re already seeing extensive loss of corals at the time of peak bleaching,” he said. “It’s heartbreaking to see damage as severe as this as soon as this.”

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Bleached acropora coral and algal overgrowth about 10 metres below the surface. Photograph: Grumpy Turtle Films

Hughes said every part of the reef system had now bleached at least once since 1998. Some reefs had bleached three or four times. He said the cumulative damage made it harder for reefs to recover and more likely they would succumb.

The Intergovernmental Panel on Climate Change in 2018 found that most tropical coral reefs would be lost if global heating was limited to an average of 1.5C above pre-industrial levels and 99% were likely to be lost of heating reached 2C. They found they would be at high risk at 1.2C, a level that may have already been reached.

Dr Lissa Schindler, an ecologist and the reef campaign manager with the Australian Marine Conservation Society, called on the Great Barrier Reef Marine Park Authority to release maps showing the extent and severity of the bleaching so the public had a true picture of the scale of the impact.

Schindler also urged the authority, which she described as the reef’s custodian, to play a greater role in advocating for stronger action on emissions.

She said in the past the authority had called for “strong and fast national action” to deal with the climate crisis, but a more recent climate statement it issued focused on global action and did not say anything about Australia needing to increase what it was doing.

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“If the Albanese government is serious about its commitment to Unesco to protect the reef then it must commit to net zero emissions by 2035 and stop approving new fossil fuel projects,” she said.

Scientists have said the government’s emissions reduction targets – a 43% cut compared with 2005 levels and net zero by 2050 – are consistent with global action that could lead to 2C of global heating.

Interviewed on ABC’s Radio National on Wednesday, the environment minister, Tanya Plibersek, said the government was “very concerned about the bleaching that we see at the moment, sadly, not just on the Great Barrier Reef, but right around the world”.

She said the government was doing “whatever we can” to get to net zero emissions. “We need to protect the reef because it’s … unique in the world and also 64,000 people rely on it for their work,” she said.

A reef authority spokesperson said it confirmed in early March that widespread coral bleaching was unfolding. They said in-water coral surveys were continuing and a report including data and maps would be published “in the coming weeks”.