"It's pretty obvious now with three flights to go that this event is certainly not a minor one," Professor Hughes told the Herald and The Age. "It certainly makes it three big events in five years." While the full severity of the bleaching will take some time to determine, the most popular visiting areas of the reef may have been spared. Bleaching on the Great Barrier Reef corals near Cooktown, as taken last week by a survey team led by James Cook University's Professor Terry Hughes. Credit:Via Terry Hughes "There's good news for the region's tourist folk," Professor Hughes said, with the Whitsundays "less bleached than reefs further offshore" and the Cairns-Port Douglas region also faring relatively well. Corals rely on algae known as zooxanthellae to provide the bulk of their energy and much of their vibrant colour. When exposed to sustained abnormal heat - measured in so-called degree-heating days - corals begin to expel the algae, leading to mass bleaching.

Coral bleaching at Magnetic Island March 2020 - part of another big bleaching event across large parts of the Great Barrier Reef. Credit:Victor Huertas/ARC Centre of Excellence for Coral Reef Studies How many of the corals recover from the bleaching or just die will take time to play out, and hinge on whether water temperatures return to average temperatures. "If the bleaching is mild, they will regain their colour and regain their vitality," Professor Hughes said. Record February heat over the reef set up the bleaching risk, although the arrival of Cyclone Gretel in the Coral Sea this month helped stir up the ocean as well as bring sustained cooling rain and cloud cover.

Scott Heron, an environmental physicist and associate professor at James Cook University, said that, because the southern reef escaped heat stress in 2016 and 2017 unlike the northern and central regions, this bleaching event could have a great impact on its vulnerable corals. "Branching and table corals grow quickly but have lower heat tolerance than other corals, like boulder corals [which are still affected by bleaching]," Professor Heron said. The warming ocean is affecting reefs worldwide, including off the coast of Brazil, in parts of Melanesia and Indonesia, he said. Sea-surface temperatures reached record-high levels on the Great Barrier Reef, topping those in 2016 and 2017 when there was mass coral bleaching. Credit:Bureau of Meteorology "The key thing with the Great Barrier Reef this year is that temperature anomalies across the reef have been spatially consistent - that is to say it’s just hot everywhere," Professor Heron said.

"In previous bleaching events we have seen a focal area of heat stress," he added. "This year we have seen heat stress from the Torres Strait down to the Capricorn Bunkers. This is global warming realised." Loading Tony Fontes, an independent dive instructor at Airlie Beach, confirmed the Whitsundays were one of the few regions where reefs were looking relatively healthy, despite a small amount of bleaching that wouldn’t be visible in aerial surveys. Wave action during Cyclone Debbie in 2017 destroyed significant amounts of coral, so many tourism operators had already relocated their dives sites. The tourist economy, though, has lately been smashed by the COVID-19 pandemic, which has all but closed the reef to visitors, Mr Fontes said.

"When we come through at the other end, tourism is going to need all the support it can get but most importantly, tourism needs a healthy reef," he said. Longer term, though, unless governments moved away from backing fossil fuels and boosted help for renewable energy as national economies emerge from the virus impact, the future of the reef would remain in peril, he said. Loading Replay Replay video Play video Play video "To protect our Great Barrier Reef, we need to transition off fossil fuels, particularly coal, that’s the crux of the problem," Mr Fontes said. "Coral bleaching is caused by global warming."

Sussan Ley, the federal environment minister, said the government had been monitoring the latest Reef bleaching events closely with the Great Barrier Reef Marine Park Authority (GBRMPA), including with the agency's role as an observer on aerial surveys. "It is deeply concerning the Reef has suffered another bleaching event with many areas of moderate to severe bleaching," Ms Ley said. "Across the Reef, the bleaching appears different from 2016-17 in that, while it is widespread, we are seeing mixed areas of severity within regions including pockets that are unaffected," she added. "Thankfully, some of the most recognised tourism areas have been less impacted." Late on Thursday, the Marine Park Authority issued a statement confirming "mass bleaching is occurring on the Great Barrier Reef, with very widespread bleaching detected". Impacts varied from no bleaching to the most severe level, with some southern areas of the Reef that had little or no bleaching in 2016 and 2017 now experiencing moderate or severe bleaching, it said.