Highest ocean heat in 400 years poses 'existential threat' to Great Barrier Reef

The Great Barrier Reef recently experienced the highest ocean temperatures in at least four centuries and faces an "existential threat" due to repeated mass coral bleaching episodes, a study published Wednesday in Science found.

The network of coral reefs off of Australia—the world's largest living structure—has faced five of the six hottest three-month periods of average surface temperature ever recorded just since 2016, each of which was accompanied by devastating coral bleaching.

Ocean temperatures around the reef reached a record-breaking extreme from January to March this year, with the three-month mean temperature 1.73°C higher than the pre-1900 average, according to the study, authored by researchers based in Australia.

The study includes climate modeling that attributes the temperatures to fossil fuel-driven carbon emissions, and concludes that urgent climate action is needed.

"This attribution, together with the recent ocean temperature extremes, post-1900 warming trend, and observed mass coral bleaching, shows that the existential threat to the Great Barrier Reef ecosystem from anthropogenic climate change is now realized," the study says.

"In the absence of rapid, coordinated, and ambitious global action to combat climate change, we will likely be witness to the demise of one of Earth's great natural wonders," the authors also wrote.

The researchers estimated the surface temperatures for 1618-1899 by using a reconstruction method based on drilling into coral skeletons and analyzing the chemical makeup. For the period from 1900 to 1995, they used both the reconstruction method and measurements by modern instruments, and for the last 30 years they used instrumental data.

They found that temperatures were relatively stable until 1900 but have climbed steadily since, especially since 1960.

The trend has culminated in a series of bleaching events, in which stressed corals expel the microscopic algae in their tissues and become transparent or white. Without the helpful algae, which live inside them symbiotically, corals are at risk of disease and death.

In interviews with journalists, the study authors spoke about the severity of the threat to the Great Barrier Reef and the urgent need for climate action.

"The heat extremes are occurring too often for those corals to effectively adapt and evolve," Ben Henley, a paleoclimatologist at the University of Melbourne and lead author of the study, toldThe New York Times. "If we don't divert from our current course, our generation will likely witness the demise of one of Earth's great natural wonders, the Great Barrier Reef."

Henley said he snorkeled with his father on the Great Barrier Reef as a child.

"You can't even take in the diversity," he said. "It's a kaleidoscope of color, it's absolutely spectacular."

He said he worries that his own 2-year-old daughter may not be able to enjoy the same experience.

"In her childhood years the reef is likely to see immense destruction," he said.

He called for strong global action so that his daughter and members of her generation could "marvel at the reef in their lifetimes."

Helen McGregor, a scientist at the University of Wollongong and study co-author, told the BBC the new research "could send a huge signal to the world about how grave the problem is."

"We know what we need to do," she added. "We have international agreements in place [to limit global temperature rise]."

Scientists not involved in the study agreed about the importance of the research, not just for the Great Barrier Reef but for coral reefs more generally.

"It's a stunningly important summary of the history of the world's largest reef system," Stephen Palumbi, a marine biologist at Stanford University, told the Times. "The paper lays out the danger that corals all around the world face from this heat."