Australia's Great Barrier Reef has been hit by severe coral "bleaching" for the second year in a row in the face of record-breaking temperatures, scientists have said.
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Bleaching happens when algae that lives in the coral is expelled due to stress caused by extreme and sustained changes in temperatures, turning the coral white and putting it at risk of dying if conditions do not return to normal.
Researchers said the back-to-back bleaching was being driven by climate change and the window was closing fast to cut the greenhouse gas emissions pushing up temperatures and harming the reef.
Aerial surveys along the entire length of the Great Barrier Reef show two-thirds suffered severe bleaching, with the middle third of the reef suffering the most intense damage and only the southern third unscathed.
This year's problems follow severe bleaching last year, which was worst in the northern third of the reef.
Last year's mass bleaching was driven in part by the "El Nino" weather phenomenon in the Pacific, which pushes up temperatures, but this year's problems occurred even without the effects of El Nino.
With it taking a decade for corals to fully recover from damage, the back-to-back bleaching left "zero prospect" of recovery for reefs hit last year, the experts warned.
The Great Barrier Reef is the world's largest collection of coral reefs, with 400 types of coral, 1,500 species of fish and 4,000 mollusc species, and is the habitat of wildlife such as the dugong, or sea cow, and the large green turtle.
Terry Hughes, director of the Australian Research Council Centre of Excellence for Coral Reef Studies, who undertook the aerial surveys both last year and this year, said: "The combined impact of this back-to-back bleaching stretches for 1,500 km (900 miles), leaving only the southern third unscathed.
"The bleaching is caused by record-breaking temperatures driven by global warming.
"This year, 2017, we are seeing mass bleaching, even without the assistance of El Nino conditions."
Dr James Kerry, who also undertook the aerial surveys, 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 offers zero prospect of recovery for reefs that were damaged in 2016."
The scientists also warned that Tropical Cyclone Debbie hit a corridor of the Great Barrier Reef at the end of March, which is likely to have caused varying levels of damage, on a section of the reef that had largely escaped the worst of the bleaching.
Prof Hughes said: "Clearly, the reef is struggling with multiple impacts.
"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: 1C 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."