Coral bleaching on the Great Barrier Reef.

Coral bleaching on the Great Barrier Reef.Credit:Tom Bridge/ www.tethys-images.com

Professor Hughes of the Australian Research Council Centre of Excellence for Coral Reef Studies said coral bleaching events,which are driven by hot weather and clear sunny days,now occur on average every six years and their frequency is set to rise.

The federal government has committed $100 million for reef restoration projects,beginning with $2 million to trial underwater fans to cool the water for a tiny proportion of the reef and is also investigating using sun shields to reduce solar radiation as well as funding ‘coral gardening’ to grow and replace dead corals.

“Restoration projects are a distraction from the real work required to save the reef,” Professor Hughes said.

“The story that clever science can fix it has no credibility. These projects are feasible,but only at a very small scale at great expense. We need to address the root cause,which is climate change.”

Professor Terry Hughes of the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University.

Professor Terry Hughes of the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University.Credit:Jason South

When bleached,corals are highly stressed but not dead. However,they are more likely to die when they’re bleached and take at least 10 years to recover - depending on the species. Some large colonies of longer-lived species are hundreds of years old. These species are generally more resistant to bleaching than faster-growing species,but with the increased severity of bleaching events,even these ‘tougher’ species are being lost.

Overheated sea surface temperatures cause corals to eject the algae which live in their tissue,which causes them to turn stark white.

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“The first time we saw back-to-back events on the Great Barrier Reef was the bleaching in 2016 and 2017,” Professor Hughes said. “Now it’s predicted that back-to-back bleaching could occur every year by mid-century.”

The Centre of Excellence for Coral Studies found last year the number of corals in the Great Barrier Reef hashalved in the past twenty years.

Increasingly frequent bleaching events are occurring too close together and corals don’t have enough time to recover.

Increasingly frequent bleaching events are occurring too close together and corals don’t have enough time to recover.Credit:XL Catlin Seaview Survey

These centuries-old corals also provide a valuable record of climatic conditions. Like tree rings,corals show distinct annual ‘growth bands’ which can be used to reconstruct environmental conditions over hundreds of years,including past bleaching events. They reveal that mass bleaching events of the scale we are currently seeing did not occur prior to the 1980s.

Professor Hughes said the narrowing window for coral recovery had already closed too far for coral reefs to bounce back to full health.

“In the past decade after 2010 the gap between bleaching events had dropped to just six years - and that gap is the window for coral recovery,” he said. “The problem is corals grow pretty slowly. They need a decade for recovery of the faster-growing species,and much longer for slower-growing ones.”

Centre of Excellence senior research fellow Tom Bridge said global warming meant large-scale climate events like a strong El Nino were no longer required to induce bleaching.

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“In the past year there has been bleaching events around the world,including on the Great Barrier Reef and the Coral Sea over summer,in Taiwan during the northern summer,and currently in the Solomons Islands and Papua New Guinea,” said Dr Bridge,who is also a senior curator of corals at the Queensland Museum Network.

“These bleaching events are occurring during La Nina events,which usually produce cooler conditions.”

“There is now enough heat in the system that any given summer is hot enough to induce a bleaching event. We’re relying on weather events,such as monsoonal troughs or cyclones,to come at the right time to reduce heat stress and avoid bleaching. Even cooler areas of the subtropics are seeing repeated bleaching from these marine heatwaves.”

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