For the second year in a row, severe coral-bleaching has been reported across Australia's Great Barrier Reef, according to helicopter surveys covering 8,000 kilometers, published Monday. Last year instances of bleaching occurred just in the northern regions, but this year they have reportedly hit the middle interior. Only the lower third of the 2,300-kilometer-long reef remains unscathed, say scientists, who point to record-breaking seawater temperatures as the cause.
The Great Barrier Reef is the world's largest coral reef system, made up of almost 3,000 individual reefs and 900 islands. As well as being a World Heritage site, it is deemed by many conservationists to be one of the most biologically rich and productive parts of planet Earth.
"It really was shocking to see the similar levels of severe bleaching this time in the central Great Barrier Reef that I saw last year in the northern third of the reef," said Dr. James Kerry, part of the team at the Australian Research Council's center of excellence for coral reef studies. "Some of the sections ... have had a double dose of severe bleaching for two years in a row, and of course, on those reefs, the corals would have effectively zero possibility of having had any recovery."
Bleaching events occur when the photosynthetic algae that coexist within coral lose their pigmentation or die-back, resulting in the coral starving – often gradually – over many years. The algae within this coral can re-colonize, but this takes time. So far, well-documented bleaching events like these new results show have hit Australia's Great Barrier Reef four times: in 1998, 2002, 2016 and now in 2017.
"It's important to remember that a bleached coral is not necessarily a dead coral," said Kerry. "There are prospects for recovery. But, in the central region, on the worst-affected reefs, we would expect to see quite substantial coral loss in the coming months."
Jörg Wiedenmann, professor of biological oceanography at the University of Southampton, told VICE News: "Coral reefs are very sensitive to environmental change. Apart from heat stress, they respond also to other stressors such as sediment run-off, nutrient enrichment, and overfishing. As such, they are indicators of the increasing pressure that humans exert on ecosystems on the regional and global scale."
The real concern for many scientists is in the back-to-back nature of the bleaching in 2016 and 2017 – a "double whammy," according to the scientists involved in the study.
"Under good conditions, it takes a reef about a decade to recover," said Wiedenmann, "provided that there are no further problems such as low-water-quality issues, repeated heating events and that there are enough corals left to seed the depleted reefs."
Whether the reef will be afforded protection from future bleaching seems, at present, unlikely. Like a boxer seeing stars, parts of the coral reef are, it seems, on the ropes. And unlike last year, this year's bleaching occurred without the assistance of El Niño conditions.
"Clearly the reef is struggling with multiple impacts," Terry Hughes, a professor and lead author of the ARC study, explained. "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."
"Ultimately, we need to cut carbon emissions, and the window to do so is rapidly closing."
An Australian government report in 2012 estimated that the value-added economic contribution generated in the Great Barrier Reef catchment was $5.7 billion (£3.44 billion). About 69,000 people are employed in fishing and tourism services related to the reef.
Jules Howard is a zoologist, author, and freelance science writer.