Great Barrier Reef threatened by unprecedented sea heat extremes, say scientists

Recent heat extremes in the seas around Australia’s Great Barrier Reef are “unprecedented” in the past 400 years – and driven by climate change, scientists have said.

Researchers examining high sea temperatures in the Coral Sea where the reef lies, which have led to repeated and devastating coral “bleaching” events since 2016, said heat extremes seen in the past decade are the hottest in data going back four centuries.

And modelling which compared scenarios with and without human activity such as putting out greenhouse gas emissions from burning fossil fuels, showed the sea’s warming trend in recent decades and the latest extremes are down to climate change.

The researchers warned that without rapid action to tackle climate change, “our generation will likely witness the demise of one of Earth’s great natural wonders”.

The Great Barrier Reef is the world’s largest coral reef system and supports a huge diversity of life, from whales and dolphins to 1,500 fish species and threatened creatures including the large green turtle and the sea cow (dugong).

The reef, which was listed as a World Heritage Site in 1981, is a major tourism draw for Australia, and an important marine and cultural resource for Australian Aboriginal and Torres Strait Island peoples.

But it has been hit by recent high sea surface temperatures which have led to repeated mass coral bleaching events between 2016 and 2024, which kill coral and threaten the survival of the reef and the wildlife that relies on it.

In a study published in the journal Nature, a team led by scientists from Australia used records from 1900 and data from core samples from corals – some of which have lived for 400 years – to reconstruct the sea temperatures back to 1618.

They said that the January-to-March Coral Sea heat extremes in 2024, 2017 and 2020 were respectively the warmest in 400 years, with this year’s heat an estimated 1.73C above the pre-1900 average.

The 2016, 2004 and 2022 events were the next warmest, the research found.

The research also found that sea temperatures were variable but relatively stable over the centuries before 1900, but warming in the industrial era has been evident since then.

Modelling confirms humans were responsible for the rapid warming in recent decades, which with recent ocean temperature extremes, the post-1900 warming trend and the mass coral bleaching, shows the “existential threat” to the reef from human-driven climate change is being realised, they said.

Dr Benjamin Henley, from the University of Wollongong and University of Melbourne, said: “The Great Barrier Reef is one of the most globally ecologically significant places on Earth, and it is a vast and spectacular natural wonder.

“However, we’ve seen in recent years these mass coral bleaching events have driven some devastation on the reef.”

He said the study looked to set the long term context for the recent extremes, by reconstructing 400 years of January-to-March sea surface temperatures in the Coral Sea, using coral records drilled in the region and slightly beyond.

“The findings of the study are that the recent events are extreme in nature, in terms of the last four centuries, unprecedented events,” he said.

“We also found, using climate models, that climate change is responsible for the warming and for these extreme sea surface temperatures.”

He warned that putting all the evidence together, the mass bleaching events, the record temperatures, the fact climate change was responsible, and the knowledge of future climate projections and that heat extremes were happening too often for the corals to adapt, showed “the reef is in danger”.

“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,” he said.