"We& 're at a point where people've ever lost near half the corals from shallow-water habitats across the northern two-thirds of this Great Barrier Reef due to backtoback bleaching over two years", mentioned Sean Connolly, also with the center for coral reef research at James Cook University.
The thresholds "are lower than we thought they would be", Hughes said.
The outcome is not just outward - zooxanthellae are partners with coral in an ancient symbiotic relationship, conducting photosynthesis necessary for the corals to survive.
In the world's largest coral system off the Australian coast, the scientists found that a marine heat wave in 2016 prompted catastrophic levels of bleaching and, when the reefs didn't recover, die-offs.
"We've been saying for a long time that these bleaching events are going to get more frequent and the effect of that will be that the mix of species will change", Professor Hughes said.
"We're in unchartered territory", Professor Hughes said, adding, "Where we end up depends completely on how well or how badly we deal with climate change". Others were killed more slowly, after their algal partners were expelled.
The scientists mapped the geographical pattern of heat exposure from satellites, and measured coral survival along the 2,300-km length of the Great Barrier Reef following the extreme marine heatwave of 2016. This algae - which also gives coral its lovely colour - is put under stress by rising ocean temperatures.
The paper says extreme weather events, caused by man-made climate change, are "rapidly emerging as major contemporary threats to nearly all ecosystems", and suggests that while not all corals will be killed off, the shape and variety will be affected.
One widely circulated tweet from Hughes amid the frantic research captured the mood: "I showed the results of aerial surveys of" bleaching on the Great Barrier Reef "to my students, And then we wept", he wrote.
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Police said the expect to meet with the prosecutor's office Thursday to determine additional charges. The couple, from Michigan City, were in Merrillville for a medical appointment, police said.
Scientists said that if nations honored global commitments in the Paris climate accord aimed at preventing temperatures from rising more than 2 degrees Celsius, Australia would still have the Great Barrier Reef in 50 years.
"They are the ones that are going to refeed and repopulate an altered reef into the future", Hughes said, adding that one way to maintain their health was through improving water quality by reducing coastal pollution.
With year-round temperatures of 29 - 30C, the Red Sea is a more thermally stable environment than many other reef communities.
Most of what researchers know about reef recovery has come from the study of the aftermath of cyclones.
Some species, such as staghorns and tabular corals, were particularly susceptible, while dome-shaped porites corals were relatively resilient. "Rather, temperature-sensitive species of corals began to die nearly immediately in locations that were exposed to heat stress".
Prof Hughes warned work needs to start urgently to protect the corals which are left.