Coral Sea's hottest summer in 400 years puts Great Barrier Reef on verge of 'in danger' status again (2024)

Sea surface temperatures in waters surrounding the Great Barrier Reef this year reached their warmest levels in more than 400 years, new research has found.

The findings of the long-term study suggest human-caused climate change is the driving factor behind increasingly high summer temperatures in the Coral Sea, which puts corals under stress and can spur mass bleaching.

Researchers warn the findings show the Great Barrier Reef is being pushed closer to a tipping point from which it may not recover, and that urgent action is needed to limit the global temperature rise to a maximum of 1.5 degrees Celsius.

Coral 'like tree rings'

The research, published in the journal Nature, reconstructed historic sea temperature data dating back as far as the year 1618.

That data was obtained by drilling into the cores of coral colonies and analysing their structure.

"They form bands, they change in density, and they're a bit like tree rings that we can count," said Professor Helen McGregor, a geologist and climate change researcher at the University of Wollongong.

Professor McGregor said the ocean temperatures were ascertained using fine-scale measurements to look at changes in the amount of strontium relative to calcium in the coral skeleton, along with changes in oxygen isotopes.

Using that data and instrument recordings dating back to 1900, the researchers ranked sea surface temperatures in the Coral Sea by year from warmest to coolest.

This year smashes records

They found the period from January to March 2024 was the warmest in more than four centuries, followed by the summers of 2017, 2020, 2016, 2004, and 2022.

"It's head and shoulders above even the latest record in 2017 for the Coral Sea," said lead author Dr Benjamin Henley of the severity of the 2024 temperature.

Dr Henley said the findings were not only scientifically significant but "of great sadness" as they reveal the world is in danger of losing one of its most spectacular natural wonders, the Great Barrier Reef.

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"I find that to be an absolute tragedy," the University of Melbourne lecturer said.

"It's hard to understand how that can happen on our watch in our lifetime."

It is the first time researchers have been able to compare recent warm summers to those dating back centuries, and Dr Henley said the warm years over the past decade were "very extreme in that longer-term context".

Reef teeters on 'in danger' status

The reef experienced its fifth mass bleaching event earlier this year which the Australian Institute of Marine Science has described as "one of the most serious and widespread so far".

For years, the Great Barrier Reef has been at risk of being placed on a list of UNESCO World Heritage sites considered "in danger" of losing its status.

Last month the global committee ratified a recommendation not to place the reef on that list, but UNESCO will require Australia to provide a progress report next year on the actions it had taken to improve the reef's health.

Corals can recover from bleaching events once sea temperatures cool, but extended or repeated severe bleaching can kill them, particularly if they are also under stress from pollution or acidification.

The authors of the Nature study said assessments of the Great Barrier Reef's "in danger" status should take a multidisciplinary approach and consider its physical, biological, and human drivers.

"Clearly climate change is the long-term driver of the stress to these coral reef ecosystems so, in our view, the Great Barrier Reef is in danger," Dr Henley said.

University of Queensland Professor Ove Hoegh-Guldberg, a climate scientist who specialises in coral reefs, said the trajectory of rising temperatures, if unchecked, would lead to coral disappearing from reefs within only a decade.

"That tipping point idea — where everything looks like it's going fine and then collapses — I think is relevant to this discussion," he said.

The authors say that even under a 1.5C warming scenario more than 70 per cent of corals across the globe could be lost, with reefs to become less species rich in the future.

However, the study's authors said hope was not lost with climate action at the local and global scale critical.

"I think we can do it but we're not quite there mentally, and I think that's what has to really change," Professor Hoegh-Guldberg said.

Concerns for coastal towns

Dr Henley said the study's method could be used on other reefs around the world, provided they are home to large enough corals capable of producing continuous long-term records.

Professor McGregor said the findings were significant for coastal communities as well as coral reefs.

"If you are living within a few tens of metres above the current sea level you should be really, really worried about this because the consequences of warming are so clear," she said.

"And we also know that from looking at the geological record.

"We know 125,000 years ago global temperatures were 2-3C warmer than today, and the sea level was at least 6 metres higher.

"Why we are not doing everything we can to avoid that is just incredibly frustrating."