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Coral bleaching on Stanley Reef,south of Townsville,which occurred in a late summer heatwave in March.Credit:Australian Marine Conservation Society,Harriet Spark

The Australian Institute of Marine Science’s annual scorecard found hard coral cover is near its highest levels since records began in the mid-1980s.

This measure,which shows corals are regrowing after four mass bleaching events since 2016,is often seized upon by climate change sceptics to play down the risks to the reef.

But the institute has cautioned that most of the increase in hard coral cover since 2016 is attributable to only a handful of fast-growing species such as table and staghornAcropora,which often suffer high death rates but bounce back quickly from bleaching events and cyclones.

Critically,hard coral cover does not measure the biodiversity of the reef,whether rare species are being lost,or other important changes.

“The reef remains a wonderful,complex and beautiful system,but it is at increased risk with climate change driving more frequent and severe bleaching events,putting increasing pressure on the ecosystem’s resilience,” the institute’s research program director,David Wachenfeld,said.

The institute’s senior research scientist,Mike Emslie,said while the reef had shown an innate ability to recover from cyclones and bleaching,the trend pointed towards escalating consequences.

“The worrying signal in the data that we have is that now these disturbances are starting to come more frequently,they’re starting to be more widespread and they’re starting to be more severe,” Emslie said.

Bleaching occurs when the sea surface temperature is too hot for too long,causing corals to expel the algae living in their tissues and turn white. Scientists say global warming is the major factor driving more frequent bleaching events.

The Bureau of Meteorology says anotherEl Nino system,which typically brings hotter,drier conditions to the eastern seaboard,could develop this summer. The last one coincided with a widespread mass coral bleaching event in 2016.

“The Bureau of Meteorology has already declared abnormally high temperatures in the waters of the Great Barrier Reef during winter,” Emslie said.

“I don’t like to make predictions,but if things unfold the way they did before in 2016,then we could be in for something similar to that.”

Earlier this month,Australia was granted a reprieve from a threat to downgrade the World Heritage status of the Great Barrier Reef following the Albanese government’s upgraded climate target. However,the United Nations Educational,Scientific and Cultural Organisation said Australia “should be required” to demonstrate progress on protecting the reef next year.

Another mass bleaching event this summer could force UNESCO’s hand,which would be damaging for the government at a time when it is being pressured by its left flank to raise Australia’s climate target.

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