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Record ocean temperatures have dealt a devastating blow to one of the world's greatest natural wonders: coral bleaching on the Great Barrier Reef has reached "catastrophic" levels, a new study suggests. More than 50 percent of corals studied near one of the reef's southern islands died last year in the most severe and widespread bleaching event ever to hit the region, Australian scientists say.
Factors that caused coral bleaching
In 2024, the reef experienced its worst summer on record. Rising ocean temperatures broke all previous records, triggering the seventh mass coral bleaching event. Coral bleaching occurs when marine heat waves stress corals, causing them to expel algae from their tissue, resulting in a loss of color. This damaging process weakens corals, making them vulnerable to disease and death.
The main cause is the ongoing burning of fossil fuels, which contributes to global warming. Additionally, the damage was compounded last year by the El Niño weather phenomenon, known for raising ocean temperatures in the region. As a result, areas of the reef that had never experienced such intense heat before were severely affected.
Mass death and spread of disease among corals
Scientists from the University of Sydney monitored 462 coral colonies on Wan Tree Island for five months, starting in February, when the heatwave peaked. By May, 370 of these colonies had bleached, and by July more than half of the bleached corals had died. Some coral species had mortality rates of up to 95%, and the researchers observed a phenomenon called “colony collapse,” in which the coral skeleton detaches from the reef and collapses. Goniopora species suffered even more, contracting black band disease, which penetrates the coral tissue and is often fatal.
“Our results highlight the urgent need to protect coral reefs, which are not only hubs of biodiversity but also vital for food security and coastal protection,” — said the lead author of the study, Maria Byrne from the University of Sydney.
Byrne also noted that the study area is in a remote, protected area of the reef, far from tourist or industrial influences. However, she said, the reef, “Despite its protected status, it was unable to escape the extreme heat stress that triggered this catastrophic bleaching event.”
The Great Barrier Reef is under threat
The Great Barrier Reef, covering almost 345 square kilometres, is the world's largest coral reef system. It is home to over 000 species of fish and more than 1 species of hard coral. Its importance to the Australian economy, particularly in the tourism sector, highlights the reef's global importance and the need for action to protect it.
However, recent bleaching episodes indicate a worrying trend: heat waves are becoming more frequent and intense, making it harder for corals to recover. The study authors warn that mass bleaching has become “an event that occurs every two years”, and emphasize the need “urgent global action to achieve ambitious climate goals and reduce emissions.”
Spread of the crisis to new territories
The latest bleaching has also affected parts of the reef that previously escaped mass impact, suggesting the crisis is spreading to new areas. Even coral species that were thought to be resilient are starting to show signs of disease and increased mortality.
“Seeing the effects on a reef that has largely avoided mass bleaching until now is truly horrific,” — said marine scientist and study co-author Shawna Fu. “The high mortality rates and spread of disease, especially in such a remote and pristine area, underscore the seriousness of the situation.”
Climate threat to coral reefs
Severe bleaching events have been observed on the Great Barrier Reef in 1998, 2002, 2016, 2017, 2020, and 2022. The 2022 episode is of particular concern because it coincided with a La Niña event—the cooler opposite of El Niño—raising further doubts about the reef's ability to withstand future climate extremes.
The new findings highlight the reef's fragile state and the urgent need to reduce fossil fuel emissions that contribute to global warming. With bleaching now looking inevitable in many places, the survival of coral reefs depends on global efforts to curb rising temperatures. It offers hope that the reef's vibrant ecosystems can still be preserved for future generations. The study is published in the journal Limnology and Oceanography Letters.
