Parts of Australia’s iconic Great Barrier Reef, famed for its stunning colors and biodiversity, suffered the biggest decline of coral on record last year after a marine heatwave bleached vast swathes of hard coral, a new report has found.
The reef had its worst summer on record in 2024 when the world underwent a rare global mass coral bleaching event impacting dozens of countries. Surging water temperatures combined with an El Niño weather pattern caused stressed corals to expel algae and lose their color.
An official analysis released on Wednesday now shows the reef lost between a quarter and a third of its hard coral cover across three main regions, according to the Australian Institute of Marine Science (AIMS). In some areas, particularly hard-hit reefs lost up to 70% of their living coral.
The 2024 bleaching event “had the largest spatial footprint ever recorded on the (Great Barrier Reef),” the report found. “The declines in the north and south were the largest in a single year since monitoring began 39 years ago.”
CNN witnessed the widespread bleaching in visits to the Great Barrier Reef in 2024 – and to the Ningaloo Reef on Australia’s western coast in 2025 – during which one scientist described the devastation as like “wildfires underwater.”
The massive loss across the 2,300-kilometer (1,400-mile) ecosystem is especially discouraging after it experienced significant growth in the last few years, with some fast-growing corals that helped the reef recover from previous bleaching.
But these corals “were known to be extremely vulnerable and one bad summer away from losing those gains,” the report said – adding that those fears came true last year, with the fast-growing corals among the most severely impacted.
“It raises the prospect that the Great Barrier Reef may reach a point from which it cannot recover,” it warned.
Marine devastation
Covering nearly 133,000 square miles (345,000 square kilometers), the Great Barrier Reef is the world’s largest coral reef, home to more than 1,500 species of fish and 411 species of hard corals.
It contributes billions of dollars to the Australian economy each year, mainly through tourism, and is promoted heavily to foreign visitors as one of the country’s – and the world’s – greatest natural wonders.
But all that is in danger, with activists and scientists warning with increasing urgency over the past decade that the reef’s future is at risk.
Severe mass bleaching at the Great Barrier Reef had previously been observed in 1998, 2002, 2016, 2017, 2020 and 2022. Corals can recover from bleaching if the temperatures return to normal, but they will perish if the water stays warmer than usual.
Water temperatures spiked again early this year during Australia’s summer, peaking in March and causing even more bleaching – though its impacts have not yet been fully measured and were not included in this report.
Oceans are now storing 90% of the excess heat from global warming — and each of the last eight years has set a new record for the amount of heat stored in the ocean.
The destruction of marine ecosystems would deliver an effective death sentence for around a quarter of all species that depend on reefs for survival, scientists warn. Reefs also provide vital protection for coastlines, reducing the impact of floods, cyclones and sea level rise.
The bleaching is “a call to action,” said the non-profit Queensland Conservation Council on Wednesday after the report’s release.
The council, the peak body for dozens of Queensland’s environment groups, called on state and national leaders to reduce emissions and shut down coal power stations. Australian is transitioning to renewable energy, like other developed nations, but critics say it’s not happening fast enough.
“The coral reefs of the future are unlikely to look like those of the past. The loss of biodiversity seems inevitable,” the report read, urging more research into reef adaptation and protection while the world works to reduce greenhouse gas emissions.
Correction:
This story has been updated to reflect that specific regions of the Great Barrier Reef saw their biggest coral declines on record last year.