Coral Bleaching on Great Barrier Reef

Two-thirds of Great Barrier Reef hit by mass bleaching

PARTS of the Great Barrier Reef off the coast of Townsville are now considered in the "red zone" with recent aerial surveys revealing the damage of the latest mass coral bleaching event.

For the second time in just 12 months, scientists have recorded severe coral bleaching across huge tracts of the Great Barrier Reef after completing aerial surveys along its entire length.

In 2016, bleaching was most severe in the northern third of the Reef, while one year on, the middle third has experienced the most intense coral bleaching including off Townsville and south towards Mackay.

"The combined impact of this back-to-back bleaching stretches for 1,500km, leaving only the southern third unscathed," Director of the ARC Centre of Excellence for Coral Reef Studies Prof Terry Hughes said.

Great Barrier Reef coral bleaching composite map shows surveyed coral reefs in 2016 (left panel) and 2017 (right panel).
Great Barrier Reef coral bleaching composite map shows surveyed coral reefs in 2016 (left panel) and 2017 (right panel).

Prof Hughes, who undertook the aerial surveys in both 2016 and 2017, said the bleaching was caused by record-breaking temperatures driven by global warming.

"This year, 2017, we are seeing mass bleaching, even without the assistance of El Nino conditions," he said.

The aerial surveys in 2017 covered more than 8,000km and scored nearly 800 individual coral reefs closely matching the aerial surveys in 2016 that were carried out by the same two observers.
 

Aerial view of coral bleaching between Cairns and Townsville during the mass bleaching event of 2017.
Aerial view of coral bleaching between Cairns and Townsville during the mass bleaching event of 2017. Ed Roberts

Dr James Kerry, who also undertook the aerial surveys said this was the fourth time the Great Barrier Reef had bleached severely - in 1998, 2002, 2016, and now in 2017.

"Bleached corals are not necessarily dead corals, but in the severe central region we anticipate high levels of coral loss," he said.

"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."

Aerial view of coral bleaching between Cairns and Townsville during the mass bleaching event of 2017.
Aerial view of coral bleaching between Cairns and Townsville during the mass bleaching event of 2017. Ed Roberts

Coupled with the 2017 mass bleaching event, Tropical Cyclone Debbie struck a corridor of the Great Barrier Reef at the end of March.

The intense, slow-moving system was likely to have caused varying levels of damage along a path up to 100km in width.

Any cooling effects related to the cyclone are likely to be negligible in relation to the damage it caused, which unfortunately struck a section of the reef that had largely escaped the worst of the bleaching.

"Clearly the reef is struggling with multiple impacts," Prof Hughes said.

"Without a doubt the most pressing of these is global warming.

"As temperatures continue to rise the corals will experience more and more of these events: 1C of warming so far has already caused four events in the past 19 years."

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