Recent aerial surveys by the Australian Research Council’s Centre of Excellence for Coral Reef Studies have revealed only the southern third of the reef is unscathed from the bleaching events.

Researcher Professor Terry Hughes said mass bleaching happened in 2017 even without the assistance of El Nino, which normally brings warmer sea surface temperatures.

“The bleaching is caused by record-breaking temperatures driven by global warming,” Hughes said.

“Ultimately, we need to cut carbon emissions, and the window to do so is rapidly closing.”

Warmer water temperatures cause coral to expel their algae, turning them bright fluorescent colours and eventually bone white.

Marine biologist Dr James Kerry said bleached corals were not necessarily dead but it was anticipated high levels of coral would be lost in the central region of the reef, which experienced the most intense bleaching this year.

“It takes at least a decade for a full recovery of even the fastest growing corals, so mass bleaching events 12 months apart offers zero prospect of recovery for reefs that were damaged in 2016,”  Kelly said.

Tropical Cyclone Debbie also destroyed parts of the reef around the Whitsundays, a popular tourist destination that had largely escaped the worst of the bleaching so far.

While cyclones normally cause the water temperature to drop, Hughes said any cooling effects were likely to be negligible in relation to the damage caused by the slow-moving Category 4 system.

“Clearly the reef is struggling with multiple impacts,” he said.

The Great Barrier Reef is known to have experienced four bleaching events in 1998, 2002, 2016 and 2017.

– AAP