Last summer was so hot on the Great Barrier Reef, even the scientist didn’t believe it
By Mike Foley
Lifelong researcher Dr Benjamin Henley couldn’t believe his eyes when he mapped it out on paper for the first time.
Of the past 400 years, the six hottest years for sea surface temperatures on the Great Barrier Reef have all been since 2016. The summer of 2024 was such an extreme reading it was literally off Henley’s chart.
Man made climate change is driving mass coral bleaching on the Great Barrier Reef, a new study has found, with six of the hottest temperatures on record hitting since 2016. Credit: Darren Jew
He had spent years preparing his study on temperatures on the reef as the summer of 2023-2024 approached – just as weather systems were primed to deliver yet another scorching season.
“I asked the editor if they would like us to include that data point, and they said yeah,” Henley told this masthead.
“When I did that, it just plotted immediately above all the other data points in my figures. I just thought: ‘It’s got to be a mistake. Something’s wrong, what’s happened here?’
“So I re-downloaded the data, re-ran the analysis, ran it in a slightly different way, and checked about three or four times and it just stayed there. It was the warmest year in the last 400 years.
“It was a staggering finding.”
Marine heatwaves drove mass coral bleaching on the reef in 2016, 2017, 2020, 2022 and 2024.
Bleaching occurs when corals are put under severe stress due to high water temperatures, which causes the organisms to expel the algae living in their tissues and turn white.
Bleaching can kill corals, but not all that turn white die. They can recover if sea surface temperatures return to normal quickly enough. Surveys will be conducted to determine the death rate from the latest mass bleaching.
The study conducted geochemical analyses of drill core samples previously collected from the Great Barrier Reef dating to 1618 to reconstruct ocean temperatures, combined with measurements taken with instruments from 1900 onwards, to create a complete record for the past 400 years.
The study – Highest ocean heat in four centuries places Great Barrier Reef in danger – was published in the prestigious journal Nature on Thursday.
One of Henley’s co-authors, University of Wollongong Professor Helen McGregor, said the climate model analysis confirmed that human influence on the climate had caused rapid warming of the sea temperatures on the Great Barrier Reef in recent decades.
“There is no if, but or maybe – the ocean temperatures during these bleaching events are unprecedented in the past four centuries,” McGregor said.
The study’s findings indicate that the mean sea surface temperature between January and March in mass bleaching years of 2016, 2017, 2020, 2022 and 2024 were, respectively, 1.50 degrees, 1.54 degrees 1.53 degrees, 1.46 degrees and 1.73 degrees above the pre-1900 average.
It also said there is a warming trend of 0.09 degrees for the January to March period per decade from 1900-2024, and 0.12 degrees per decade from 1960-2024.
Record temperatures were set in 2016 and 2017 in the Coral Sea, and in 2020 they peaked fractionally below the record high of 2017.
Recent data indicates that a record was set earlier this year, with the mean sea surface temperature in 2024 more than 0.19 degrees above the high of 2017.
“We are facing a loss of one of the most iconic and spectacular places in the world,” Henley said.
The Intergovernmental Panel on Climate Change (IPCC) – the United Nations body for assessing global warming – has found that if global average temperatures, which are currently 1.1 degrees above the pre-industrial average, reach 1.5 degrees then 70 to 90 per cent of the world’s coral reefs will die. If warming reaches 2 degrees, it predicts 99 per cent of coral reefs die.
The Albanese government upgraded Australia’s target to cut emissions 43 per cent by 2030, up from the Morrison government’s pledge of 26 per cent by the end of the decade.
But Climate Action Tracker found that if all other countries adopted goals similar to Australia’s current policy then the world would warm by more than 3 degrees, spelling doom for coral reefs the world over.
Henley said he hopes his study pushes policymakers to make deeper cuts in global emissions.
“We can never lose hope. Every fraction of a degree of warming we avoid will lead to a better future for the human and natural systems of our planet.”
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