Ellen Phiddian
Cosmos science journalist
The Great Barrier Reef is warmer than it’s been in at least 400 years, according to a new analysis.
The temperatures recorded on the reef in early 2024, which brought a mass bleaching event, are the hottest it’s been since 1618 by a wide margin. The next 5 hottest extremes on the reef have all occurred in the last 20 years.
The study, led by Australian researchers, uses atomic signatures in corals from around the Coral Sea to construct a sea surface temperature record for the region.
The coral record stretches back to 1618, and correlates closely with direct temperature measurements beginning after 1900.
The study is published in Nature.
“The Great Barrier Reef, on the basis of our assessment, is in danger,” says lead author Dr Benjamin Henley, a lecturer at the University of Melbourne.
“The recent extreme events are extremely unusual in the long-term context.”
The researchers disagree with UNESCO’s World Heritage Committee’s decision, handed down last week, not to list the Reef as “in danger”.
“Climate change is responsible, as most people expect, and in the absence of rapid, coordinated and ambitious global action to combat climate change, we will sadly witness the demise of one of Earth’s most spectacular natural wonders,” says Henley.
The researchers developed temperature records by looking at isotopes – certain atoms that can be used as fingerprints – of oxygen stored in corals around the Coral Sea, in a region stretching from Queensland to Nauru and Fiji.
When the research was combined with 20th Century temperature records, it showed that sea surface temperatures in the region have significantly increased in the past 2 decades.
The 6 highest temperature extremes – in 2024, 2017, 2020, 2016, 2004 and 2022, in order of temperature – were all “heads and shoulders above the previous centuries”, according to Henley.
“The study’s findings, when they dawned upon us, were exciting and interesting. But when you really understand what it means in the long-term context, the feeling that comes is one of great sadness, and this sense of inevitability of the coming decades,” says Henley.
“This is one of the most precious places on earth … It actually represents life as we know it, and the world is losing one of its icons.
“I find that to be an absolute tragedy. It’s hard to understand how that can happen on our watch.”
The researchers also used computer modelling to predict what the Reef’s current temperatures would be like in the absence of human-induced global warming over the past 200 years.
“From about the 1960s, you see this strong divergence and very clear evidence that climate change is the driver of the warming that we see in the region,” says Henley.
“I get incredibly frustrated because the evidence is so clear of what is happening. And I feel sad for the individual corals that have lived for 400 years that are now under threat,” says study co-author Professor Helen McGregor, a researcher at the University of Wollongong.
“When you think of all the consequences of climate change – even as simple as sea level rise. If you are living within a few tens of metres of current sea level, you should be really, really worried about this, because the consequences of warming are so clear.”
The study has been published a day after the Australian Institute of Marine Science (AIMS) released its annual summary on reef condition.
AIMS has reported increases of coral cover on all sections of the reef this year, but emphasises that the full extent of the recent mass bleaching is yet to be realised. Because of the lag time between the bleaching of a coral and its death, next year’s survey will deliver the full verdict on how serious the recent bleaching event has been.
“On this shorter-term time scale, reefs are showing some resilience. But looking at the longer term, there are changes,” says McGregor, who is not involved with AIMS’ surveys.
The researchers say that worldwide greenhouse gas emissions cuts, in line with the goals of the Paris Climate Agreement are necessary to prevent destruction of the reef.
“There is a glimmer of hope that we can maintain or return the earth to around the 1.5° warming level,” says Henley.
“If we can do that, then there is some hope in the probabilities that we can maintain these places into the future.”
The researchers point out in their paper that global warming of 1.5°C will still likely lead to the loss of 70-90% of corals worldwide, but warming above 2°C, which is the world’s current trajectory, would “have disastrous consequences for coral ecosystems and the hundreds of millions of people who currently depend on them”.
“I remember in 1969, standing in the shallows of a beautiful coral reef, and that experience was fantastic – corals, fish, abundance,” says study co-author Professor Ove Hoegh-Guldberg, from the University of Queensland.
“I now get to 60, and that is seriously disappearing. There is a dark cloak that falls on you, in that you can’t look at a reef without worrying.
“Am I truly a pessimist? I don’t think I am, but I’m a very worried optimist.”
