Extensive coral bleaching (white/yellow patches) documented on the Great Barrier Reef during aerial surveys in March 2016. [Photo/ARC Centre of Excellence for Coral Reef Studies]?

Australia's Great Barrier Reef is currently experiencing the worst, mass bleaching event in its history, with the overwhelming majority of reefs being ranked in the most severe bleaching category, according to a statement by the Australian Research Council (ARC) Centre of Excellence for Coral Reef Studies.

It says the majority of more than 500 coral reefs from Cairns to Papua New Guinea are now severely bleached, and only four of?them have no bleaching.

"This has been the saddest research trip of my life," says Prof. Terry Hughes, convenor of the National Coral Bleaching Taskforce.

"Almost without exception, every reef we flew across showed consistently high levels of bleaching, from the reef slope right up onto the top of the reef," he added.

Prof. Hughes?points out that the severity of the bleaching event was much greater than in earlier bleaching events in 2002 or 1998.

Scientists in the water are already reporting up to 50 percent mortality of bleached corals, but Prof. Hughes says it is still too early to tell just what the overall outcome will be.

"We will continue to conduct underwater surveys along the Great Barrier Reef in the coming months as the full impact of this mass bleaching event unfolds," he said.

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.