Two-thirds of Great Barrier Reef hit by mass coral bleaching: Scientists

Says the reefs affected last year have zero prospect of recovery

Two-thirds of Australia's iconic Great Barrier Reef has been damaged by back-to-back severe bleaching events caused by record-breaking temperatures, scientists warned on Monday, adding the reefs affected last year have zero prospect of recovery.

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.

Last year, 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.

"This is the fourth time the Great Barrier Reef has bleached severely - in 1998, 2002, 2016, and now in 2017," said James Kerry from James Cook University in Australia, who undertook the aerial surveys.

"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," Kerry said.

The aerial surveys this year covered more than 8,000 kilometres (km) and scored nearly 800 individual coral reefs closely matching the aerial surveys last year that were carried out by the same two observers.

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

"The bleaching is 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," said Hughes, who undertook the aerial surveys.

Coupled with this year's mass bleaching event, Tropical Cyclone Debbie struck a corridor of the Great Barrier Reef at the end of March, researchers said.

The intense, slow-moving system was likely to have caused varying levels of damage along a path up to 100 kilometres 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," said Hughes.

"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: One degree Celsius of warming so far has already caused four events in the past 19 years," Hughes added.

"Ultimately, we need to cut carbon emissions, and the window to do so is rapidly closing," said Hughes.

Coral bleaching occurs when abnormal environmental conditions, like heightened sea temperatures, cause corals to expel tiny photosynthetic algae, called 'zooxanthellae'.

The loss of these colourful algae causes the corals to turn white, and bleach. Bleached corals can recover if the temperature drops and zooxanthellae are able to recolonise them, otherwise the coral may die.