Ocean temperatures in the Great Barrier Reef are now the hottest in at least 400 years and are an “existential threat” to the planet’s unique natural wonder, according to new scientific research.
Scientists analysed long-lived corals in and around the reef that keep a record of temperature hidden in their skeleton and matched them to modern observations.
The research, published in the prestigious journal Nature, used climate models to find the extreme temperatures of recent decades could not have happened without the extra greenhouse gases in the atmosphere caused mostly by burning fossil fuels.
The “existential threat” to the reef from the climate crisis was “now realised”, the scientists wrote, and without ambitious and rapid cuts to greenhouse gas emissions “we will likely be witness to the demise of one of the Earth’s natural wonders.”
The research comes two weeks after the world heritage committee decided not to place the reef, which covers an area larger than Italy, on its list of sites “in danger”, saying it would consider the question again in 2026.
Global heating drove a fifth mass coral bleaching event in eight years across the reef this summer that scientists at the Australian Institute of Marine Science called the most extensive and extreme on record this week.
Before the bleaching, the institute said surveys showed coral cover in the north and central parts of the reef was the highest since monitoring began in the early 1980s.
For the new study, scientists built a record of temperatures for the peak three-month January to March period for heat for all years from 1618 onwards.
The study found 2024 was the hottest in at least 407 years and 1.73C hotter than the average for years before 1900.
Dr Benjamin Henley, lead author of the research at the University of Melbourne, said: “I was shocked when I saw that data point pop out. We had to recheck it several times. It’s shocking too to realise that, right at the point, it was the warmest January to March the Coral Sea had experienced in at least 400 years.”
The five other warmest years, in addition to 2024, were 2004, 2016, 2017, 2020 and 2022. Mass bleaching events were declared in five of those six hottest years.
Bleaching is a stress reaction to heat where the symbiotic algae that give corals their colour and nutrients is lost. Corals can recover, but are more susceptible to disease and struggle to reproduce. If temperatures remain high for too long, it can be fatal.
Henley said he hoped the new findings would be considered by the Australian government and Unesco in their assessments of the risk the reef faced from global heating.
“It is our assessment that the reef is in danger,” he said.
Most of the 22 corals – from the long-lived Porites genus – used to reconstruct temperatures were outside the Great Barrier Reef, but inside the broader Coral Sea region.
Coral bleaching describes a process where the coral animal expels the algae that live in their tissues and give them their colour and much of their nutrients.
Without their algae, a coral’s white skeleton can be seen through their translucent flesh, giving a bleached appearance.
Mass coral bleaching over large areas, first noticed in the 1980s around the Caribbean, is caused by rising ocean temperatures.
Some corals also display fluorescent colours under stress when they release a pigment that filters light. Sunlight also plays a role in triggering bleaching.
Corals can survive bleaching if temperatures are not too extreme or prolonged. But extreme marine heatwaves can kill corals outright.
Coral bleaching can also have sub-lethal effects, including increased susceptibility to disease and reduced rates of growth and reproduction.
Scientists say the gaps between bleaching events are becoming too short to allow reefs to recover.
Coral reefs are considered one of the planet’s ecosystems most at risk from global heating. Reefs support fisheries that feed hundreds of millions of people, as well as supporting major tourism industries.
The world’s biggest coral reef system – Australia’s Great Barrier Reef – has suffered seven mass bleaching events since 1998, of which five were in the past decade.
Each year, corals lay down bands of dense and less dense skeleton that scientists can count to work out how old they are. The relative amounts of the chemical strontium or the type of oxygen in these bands is dependant on the temperature of the water, and so acts as a proxy thermometer.
The reconstructed temperatures were checked against coral samples taken from the Great Barrier Reef and against modern observations of ocean temperatures.
Prof Helen McGregor, of the University of Wollongong and second author of the study, said the reef would face “catastrophe” if global heating is not addressed.
She said: “The very corals that have lived for hundreds of years and that gave us the data for our study are themselves under serious threat.”
Prof Terry Hughes, a leading expert on coral bleaching at James Cook University who was not involved in the study, said: “This new study … confirms very convincingly that coral bleaching and mass mortality in the [Great Barrier Reef] region is a modern phenomenon caused by anthropogenic heating.
“It blows out of the water the persistent false claims that coral bleaching is somehow normal or cyclical.”
Prof Peter Mumby, a University of Queensland reef scientist who was not involved in the research, said the patterns of heat in recent years were now “well established”. But he said the study authors were “overly pessimistic” about the future because different parts of the reef and corals were not responding uniformly to the rising heat.
