2nd year of mass coral bleaching means part of Great Barrier Reef could die

Australia's Great Barrier Reef has faced a second mass coral bleaching event in just 12 months, which scientists fear could cause kill part of the world's largest living structure.

Scientists from the ARC Centre of Excellence reported their findings on Monday after conducting an aerial survey that covered more than 8,000 kilometres of reef off northeastern Australia. The study was published in the journal Nature.

The powerful El Nino of 2015–2016 — a periodic extreme warming of the Pacific Ocean — was partly to blame for the coral loss in 2016, scientists believe. It's estimated that 22 per cent was killed off. And part of that, they say, is due to a warming climate.

"Clearly the reef is struggling with multiple impacts," said Terry Hughes, director of the ARC centre, said in a release. Hughes was part of the survey team in the last two years.

"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: 1 C of warming so far has already caused four events in the past 19 years."

On top of the warming ocean temperature, the corals were faced with Tropical Cyclone Debbie, a Category 4 hurricane on the Saffir-Simpson scale, which affected the area at the end of March.

The concern, the researchers say, is that the coral that was damaged during the mass bleaching in 2016 has no chance of recovery.

It takes even the fastest growing coral at least a decade of good conditions to fully recover. And for that reason, they believe the two events will likely cause the death of corals in the central region.

Coral gets its colour from algae that are extremely sensitive to temperature variations. A difference of just 1 C can cause bleaching. If the temperatures do not return to normal, the colourful algae won't return and the coral can die.

The reef — which spans more than 344,400 square kilometres — has undergone similar events in recent history: in 1998, 2002, 2016 and 2017.