The aerial survey plotted the health of some 800 reefs over 8000 km of flying. Credit:ARC Centre for Excellence for Coral Reef Studies "It's been a huge blow to the reef after last year the northern third was hit and now this year's it's the middle third," Professor Hughes said. Corals bleach when temperatures exceed tolerance levels for too long, prompting them to expel the algae that supply most of their energy and the brilliant colours. Not all bleached corals die but as much as two-thirds of north corals have succumbed. "The mortality in the central region will continue to unfold over coming months," Professor Hughes said. "We've already seen substantial mortality, up to 50 per cent on some central reefs in the past six weeks." Bleaching has returned even to some of the regions hard hit last year, such as Cape York's Princess Charlotte Bay.

The central regions of the Great Barrier Reef have been hardest hit this year. Credit:ARC Centre of Excellence for Coral Reef Studies Some 30 reefs in the north had no score as they "effectively have ceased to exist at least in the shallow sections as coral reefs", said James Kerry, a marine biologist from James Cook University who also took part in the survey. While this year's event is not expected to result in as much coral mortality as in 2016, there are worrying signs corals bleached at lower temperatures than a year earlier. That result may point to more bleaching next summer if conditions are again relatively warm. "It seemed like the bleaching happened more quickly this year, which may suggest they are in a weakened, more stressed state following last year's event," Dr Kerry said.

Mark Read, manager of operations support at the Great Barrier Reef Marine Park Authority, said "2017 has shaped up to be a pretty nasty year when it comes to coral bleaching". "The corals didn't get any respite at all," Dr Read said. Bleaching, though, was patchy. The authority was already in talks with tour operators to redirect visitors to "refugia" where they can still enjoy "a high-quality experience", he said. Such areas are likely to be harder to find in the Whitsunday region near Mackay where Cyclone Debbie carved a swathe about 100km wide through the reefs. "The one area that was actually doing pretty good was the one that bore the brunt of the category 4 cyclone," Dr Read said. "A lot of those massive corals are now lying on the beach in those locations. They've simply been snapped and rolled up onto the beach."

Temperatures are showing signs of easing back to more typical levels for this time of year, such as the Davies Reef off Townsville. But the area had been remarkably warm even in a non-El Nino year, such as during the previous 2015-16 summer. In March, for instance, large areas of Queensland reported their hottest mean temperatures on record. These included the coastal region of the state's north adjacent to the Great Barrier Reef. Professor Hughes said some corals may regain their colour in the next three to six months, but it would take longer to see if they recovered their reproductive health and began a fuller recovery. The faster growing branch corals and some other rapidly growing "weedy" species of corals could rebuild the reefs but the process would take years or even a decade. The loss of boulder corals would take much longer to recover from the bleaching.

Rising temperatures from global warming means the corals are likely to enjoy ever shorter stints between mass bleaching events, the scientists said. Loading "It's highly unlikely we'll have a period of calm for 10 or 20 years that will allow these reefs to re-establish to the point that they might be recognisable as they were," Dr Kerry said. Dr Kerry said he was surprised the severity of the bleaching had not provoked more of a public response: "I don't really know what else the reef needs to do to signify that it's in serious trouble."