Aerial footage of the Great Barrier Reef between Cairns and Townsville (Photo: Ed Roberts)

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.

Surveys showed that bleaching in 2016 was most severe in the northern third of the Reef, while one year later, the middle third has experienced the most intense coral bleaching.

‘The combined impact of this back-to-back bleaching stretches for 1,500 km (900 miles), leaving only the southern third unscathed,’ says Professor Terry Hughes, Director of the ARC Centre of Excellence for Coral Reef Studies, who undertook the aerial surveys in both 2016 and 2017.

Elevated water temperatures resulting from the 2016 El Niño event and driven by global warming are thought to be responsible for the mass bleaching of 2016. ‘This year, 2017, we are seeing mass bleaching, even without the assistance of El Niño conditions,’ said  Hughes

Map of the Great Barrier Reef shows the worst affected areas in 2016/17 (Credit: ARC Centre of Excellence for Coral Reef Studies)

Coral Bleaching is a phenomenon that occurs when elevated water temperatures cause coral polyps to expel a symbiotic species of photosynthetic algae called zooxanthellae, which provides the coral with oxygen and nutrients and is also responsible for the coral’s vibrant colour. Once the algae are ejected, the mostly transparent coral loses its colour so that only the white calcium carbonate stony ‘skeleton’ is visible, hence the ‘bleaching’ effect.

The aerial surveys carried out in 2017 covered more than 8,000 km (5,000 miles), with nearly 800 individual coral reefs closely matching aerial surveys from 2016. Dr James Kerry, who also undertook both aerial surveys with Hughes, said: ’This is the fourth time the Great Barrier Reef has 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.’

‘It takes at least a decade for a full recovery of even the fastest growing corals, so mass bleaching events 12 months apart offer zero prospect of recovery for reefs that were damaged in 2016.’

Diver surveying bleached corals at Orpheus Island (Photo: Greg Torda)

Coupled with the mass bleaching event, the intense, slow-moving Tropical Cyclone Debbie, which struck a corridor of the Great Barrier Reef at the end of March, is likely to have caused varying levels of damage along a path up to 100 km wide in places. Although cooling effects from such a system can lessen the impact of the elevated sea temperatures, the scientists believe that these are likely to be negligible in relation to the damage it caused, as the cyclone unfortunately hit a section of the reef that had largely escaped the worst of the bleaching.

‘Clearly the reef is struggling with multiple impacts,’ explains 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: 1°C of warming so far has already caused four events in the past 19 years.

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