The reef is home to thousands of species, including sharks, turtles and whales. Australia relies on it for about 70,000 jobs and billions of dollars annually in tourism revenue, all now threatened by years of accumulated damage.
The study’s authors estimated how much coral had died in the immediate aftermath of the 2016 heat wave, and then returned nine months later to discern how many corals had regained their color — a sign of restored health — and how many had died. Their report describes a catastrophic die-off on the northern part of the reef, impacting the mix of coral species.
Professor Hughes said scientists had predicted a mass die-off resulting from global warming, but “what the paper shows is that it’s well underway.” He added, “That transition is happening here and now.”
Corals require warm water to thrive, but they are extremely sensitive to heat, and an increase of two or three degrees Fahrenheit above normal can kill them.
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. It would still look very different from today.
But if greenhouse gas emissions continue on their current trajectory, the reef will be unrecognizable, they 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.”
The Great Barrier Reef has bleached four times since 1998, according to scientists. Record high temperatures in 2016 were followed by another bleaching event last year.
“We’re now at a point where we’ve lost close to half of the corals in shallow-water habitats across the northern two-thirds of the Great Barrier Reef due to back-to-back bleaching over two consecutive years,” said Sean Connolly, also with the center for coral reef studies at James Cook University.