Professor Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University, said he was "gobsmacked" by the scale of the coral bleaching which far exceeded the two previous events in 1998 and 2002. Researchers inspected 84 reefs and found coral mortality rates of 50 per cent or more in the north of the Great Barrier Reef. Credit:James Cook University "It is fair to say we were all caught by surprise," Professor Hughes said. "It's a huge wake up call because we all thought that coral bleaching was something that happened in the Pacific or the Caribbean which are closer to the epicentre of El Nino events." The El Nino of 2015-16 was among the three strongest on record but the starting point was about 0.5 degrees warmer than the previous monster of 1997-98 as rising greenhouse gas emissions lifted background temperatures. Reefs in many regions, such as Fiji and the Maldives, have also been hit hard.

Bleaching occurs when abnormal conditions, such as warm seas, cause corals to expel tiny photosynthetic algae, called zooxanthellae. Corals turn white without these algae and may die if the zooxanthellae do not recolonise them. The northern end of the Great Barrier Reef was home to many 50- to 100-year-old corals that had died and may struggle to rebuild before future El Ninos push tolerance beyond thresholds. Dying corals in the Great Barrier Reef after the worst bleaching event on record. Credit:James Cook University "How likely is it that they will fully recover before we get a fourth or a fifth bleaching event?" Professor Hughes said. (Use the slider image below to see how mature staghorn corals at one reef at Orpheus Island on the Great Barrier Reef looked at low tide in 1996 prior to the 1998 bleaching event, and 20 years later.)

The health of the reef has been a contentious political issue, with Environment Minister Greg Hunt pledging more funds in the May budget to improve water quality – one aspect affecting coral health. But Mr Hunt has also had to explain why his department instructed the UN to cut out a section on Australia from a report that dealt with the threat of climate change to World Heritage sites including the Great Barrier Reef and Kakadu. '10 cyclones holding hands' Professor Hughes said tropical cyclones can cut a 50km-wide swatch of destruction of corals, but this year's bleaching event was like "10 cyclones holding hands and marching across the northern third of the Great Barrier Reef".

But cyclones can also help. The category-five Cyclone Winston that slammed into Fiji in February, brought widespread rains over parts of Queensland as a tropical depression, helping to lower sea temperatures by two degrees and sparing much of the southern corals from severe bleaching, Professor Hughes said. Even so, those southern reefs are still likely to have had their reproduction and growth rates slowed by the unusually warm seas. The scientists said the bleaching event showed how important it was to continue to bolster resilience of the reef, such as through programs to limit run-off from farms and towns bringing in excessive nutrients and pollution. "The reef is no longer as resilient as it once was, and it's struggling to cope with three bleaching events in just 18 years," Professor John Pandolfi, from the ARC Centre of Excellence for Coral Reef Studies at The University of Queensland, said. "Many coastal reefs in particular are now severely degraded." (Use the slider on image below to see mature staghorn coral at Lizard Island on the Great Barrier Reef. The corals bleached in February 2016 and were overgrown with algae by April.)

Professor Hughes said he did not advocate the reef being put on the "in danger" list by the World Heritage Committee, but it was time governments reconsidered their approval for massive new coal mines in Queensland's Galilee Basin and elsewhere. "The key threat to the Great Barrier Reef is climate change – the government has recognised that many times," he said. "[But] there is a disconnect in the policy round governments issuing permits for 60 years for new coal mines and how that might impact on the Great Barrier Reef and reefs more generally."