Australia’s iconic Great Barrier Reef is fundamentally changing because of repeated bleaching from high ocean temperatures brought on by climate change, according to marine biologists.
“It’s not a question of reefs dying or reefs disappearing, it’s reef ecosystems transforming into a new configuration,” says marine biologist Terry Hughes, from James Cook University in Townsville, Australia.
“Species like fish and crustaceans and so on — the iconic biodiversity of reefs — all depend on the structure and three dimensionality the habitat provided by corals,” Hughes says. “When you lose a lot of corals, it affects everything that’s dependent on corals.”
Corals ‘bleach’ when stressed, expelling their colourful resident zooxanthellae. According to a report released on 17 April by the Great Barrier Reef Marine Park Authority – the Australian government’s reef management agency — the World Heritage-listed reef is experiencing its worst mass bleaching event on record. The Reef Snapshot said three-quarters of the entire reef is showing signs of bleaching and nearly 40 percent is showing high or extreme bleaching.
The report is based on aerial surveys of 1,080 of the Great Barrier Reef’s estimated 3,000 individual reefs, and in-water surveys of a smaller number of reefs.
It showed that while bleaching was observed along the entire length of the Great Barrier Reef, it was most severe in the central and southern regions.
“We’ve never seen this level of heat stress across all three regions of the Great Barrier Reef,” says Brisbane-based marine biologist Lissa Schindler, from the Australian Marine Conservation Society.
This is the fifth mass bleaching event on the Great Barrier Reef in eight years. Hughes warns that climate change-driven increases in ocean temperatures are making it more difficult for the Reef’s corals to recover between bleaching events. “In the last six years, we’ve settled into bleaching every other year – in 2020, 2022, and now 2024 – and that’s simply not enough time for a proper recovery,” he says.
Global phenomenon
The Snapshot was one of a series of reports released this week on coral bleaching that also sounded alarm bells for reefs. The Australian Institute of Marine Science announced on 18 April that the Great Barrier Reef experienced water temperatures in parts of the southern reef at 2.5 degrees Celsius higher than historical summer peaks.
Meanwhile on 15 April the United States’ National Oceanic and Atmospheric Administration declared the fourth global coral bleaching event on record, and the second in the past decade. The declaration acknowledges that the warmth of the southern hemisphere summer mirrored coral bleaching events seen in the northern hemisphere summer last year.
It comes as global sea surface temperatures again broke records in 2023, associated with a strong El Niño weather pattern, recording an annual average temperature around 0.3 degrees Celsius higher in the second half of 2023 compared with 2022.
“There have been very high temperatures driven by climate change all across the world, and there has been coral bleaching in many other countries,” says environmental scientist Roger Beeden, chief scientist for the Great Barrier Reef Marine Park Authority, Townsville.
Hughes says the warming climate is pushing reefs to have less coral, and the mix of coral species is changing. For example, the branching and table-shaped corals are often the fastest to recover from a bleaching event because they are fast-growing, Hughes says. However they’re also very prone to bleaching and have higher levels of mortality during bleaching events.
“It’s a bit analogous to a fire on land through a forest, that favours a bounce-back by flammable grasses before the trees can recover,” he says. “Ironically, that that bounce-back, that resilience, undermines the ability of the reef to cope with the next inevitable bleaching event.” Seaweeds also flourish when corals degrade.
Beeden says those who live and work on the Reef are observing significant changes. “There’s historical photos that show inshore reefs that were laden with coral, and that’s very different now,” he says.
He says there are an estimated 450 different species of coral on the Reef, and such diversity means there is a chance the Reef will adapt to the changing conditions, even if it changes character. “What we see within species is definitely there is variability in how they respond to stress events.”
Hughes says the solution to the Great Barrier Reef’s bleaching problem is clear. “Reduce greenhouse gas emissions. Full stop.”