Climate change leaves two thirds of Great Barrier Reef bleached

Little of Australia's Great Barrier Reef has now been left unscathed by coral bleaching.

A recent aerial survey by scientists at the ARC Centre of Excellence for Coral Reef Studies has found the phenomenon leaving vast stretches of corals bone-white for the second year in a row.

A global bleaching event in 2016 saw an average of 67 per cent of corals lost in the north of the reef, and scientists hoped 2017 would offer the delicate ecosystem a reprieve.

The survey showed the opposite.

In 2017, scientists found the middle reef worst hit, leaving only its southern reaches untouched.

This means two thirds of the reef's corals have now been impacted by coral bleaching in a zone stretching for 1,500 km (900 miles).

[embed]https://twitter.com/ProfTerryHughes/status/851228095679836160[/embed]

Bleaching occurs when coral is exposed to stresses such as increased water temperature, causing it to expel the colour and nutrient-giving algae that lives in its tissue.

This leaves the skeleton exposed and vulnerable, making the coral more susceptible to heat, disease and pollution.

The 2016 coral bleaching event was caused by hotter water temperatures due to man-caused global warming and a strong 2015-2016 El Niño.

The 2017 bleaching has occurred without a similar El Niño effect, but in the opinion of Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies, the link between coral bleaching and El Niño has been "overstated."

"Bleaching occurs because of extreme water temperatures, which can occur with or without El Niño," he said.

Only two of the reef's four recorded bleaching events have occurred in El Niño years, in 1998 and 2016.

"It really comes down to the local temperatures," he explained.

"In the east coast of Australia, we've had a warm winter and a hot summer - March was record breaking along the east coast."

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