The reef is home to thousands of species such as sharks, turtles and whales. Australia relies on some 70,000 jobs and billions of dollars a year in tourism revenues, all of which are now threatened by years of accumulated damage.
The study's authors estimate how much coral died immediately after the heatwave in 2016 and thereafter came back nine months later to see how many corals had regained their color – a sign of restored health – and how many had died , Her report describes a catastrophic death in the northern part of the reef, which affects the mix of coral species.
Professor Hughes said scientists had predicted a mass extinction due to global warming, but "what the paper shows is that it's in full swing." He added, "This transition is taking place here and now."
Corals they need warm water to thrive, but they are extremely sensitive to heat, and a rise of two to three degrees Celsius above normal can kill them
Scientists said that if the nations met the global commitments in the Paris Climate Agreement, the aimed to prevent temperatures of more than 2 degrees Celsius, Australia would retain the Great Barrier Reef in 50 years. It would still look very different than today.
But if greenhouse gas emissions continue on their current path, they will not recognize the reef, they said.
"We are in an unknown area," said Professor Hughes, adding, "Where we land depends entirely on how well or how badly we deal with climate change."
The Great Barrier Reef has been bleached four times since 1998, scientists say. Record high temperatures in 2016 was followed by another bleaching operation last year.
"We have now reached a point where we have lost almost two-thirds of coral in shallow-water habitats over the northern two-thirds of the Great Barrier Reef due to back-to-back whitening for two consecutive years", Sean Connolly said, also with the Center for Coral Reef Studies at James Cook University.
Continue reading the main story