Warm seas have led to the death of two-thirds of a 700km (435miles) stretch of coral on the coast of Australia.
Scientists said it is the worst death or die-off ever recorded on the World Heritage site.
“The coral is essentially cooked,” professor Andrew Baird, a researcher at James Cook University who was part of the reef surveys said.
He said the die-off was “almost certainly” the largest ever recorded anywhere because of the size of the Barrier Reef, which at 348,000 sq km (134,400 sq miles) is the biggest coral reef in the world.
The news comes as an “obituary” of the Great Barrier Reef was published online pronouncing it dead.
One of the planet’s greatest living wildernesses – the Great Barrier Reef – was declared no more by leading environmental writer Rowan Jacobsen.
Mr Jacobsen wrote: “The Great Barrier Reef of Australia passed away in 2016 after a long illness. It was 25 million years old.”
The posthumous tribute to the UNESCO site which stretches for over 1.400 miles of Queensland’s north-west coast – contains 1,625 species of fish, 3,000 molluscs and 30 different types of whale and dolphin – came as scientists carried out a major investigation into the reef’s greatest enemy – bleaching.
Bleaching occurs when the water is too warm, forcing coral to expel living algae and causing it to calcify and turn white. Mildly bleached coral can recover if the temperature drops and the survey found this occurred in southern parts of the reef, where coral mortality was much lower.
While bleaching occurs naturally, scientists are concerned that rising sea temperatures caused by global warming magnifies the damage, leaving sensitive underwater ecosystems unable to recover.
UNESCO”s World Heritage Committee stopped short of placing the Great Barrier Reef on an “in danger” list last May but asked the Australian government for an update on its progress in safeguarding the reef.
Climate scientists argue that increased carbon dioxide in the atmosphere traps heat radiating from earth, creating global warming. Australia is one of the largest carbon emitters per capita because of its reliance on coal-fired power plants for electricity.
“Climate change is killing the Great Barrier Reef,” said environmentalist Charlie Wood, director of 350.org, an anti-fossil fuels movement.
“The continued mining and burning of coal, oil and gas is irreparably damaging the climate. If we want our kids to enjoy the Great Barrier Reef for generations to come, we must act now to keep fossil fuels in the ground,” Wood said in an emailed statement.