Coral bleaching has reached "catastrophic" levels on the Great Barrier Reef

Record ocean heat has taken a devastating toll on one of the world’s greatest natural wonders, with coral bleaching on Australia’s Great Barrier Reef reaching “catastrophic” levels, a new study has found. 

More than 50% of affected corals monitored near an island in the reef’s south were killed last year during the most severe and widespread bleaching to ever hit the area, according to a team of Australian scientists.

Factors leading to coral bleaching

In 2024, the reef experienced its worst summer on record. Soaring ocean temperatures shattered previous highs, causing the reef’s seventh mass bleaching event. 

Corals are bleached white when marine heat waves put corals under stress, causing them to expel algae from their tissue, draining their color. This damaging process ultimately weakens the corals, leaving them vulnerable to disease and potential death.

The underlying cause is the ongoing combustion of planet-heating fossil fuels, which is driving global temperatures upward. 

Coral damage was further accelerated last year by the El Niño weather pattern, known for raising ocean temperatures in this region. As a result, reef areas that had never endured such intense heat came under severe strain.

Extensive coral mortality and disease

Scientists from the University of Sydney tracked 462 coral colonies at One Tree Island over five months, beginning in early February when the heat wave was at its peak. 

By May, 370 of these colonies had bleached; by July, more than half of the bleached corals were gone. Some coral species suffered mortality rates of up to 95%, with researchers observing “colony collapse,” a phenomenon in which the coral skeleton detaches from the reef and disintegrates.

The Goniopora species encountered additional damage, contracting black band disease that invades the corals’ tissues and often proves fatal.

“Our findings underscore the urgent need for action to protect coral reefs, which are not only biodiversity hotspots but also crucial for food security and coastal protection,” said lead author Maria Byrne from the University of Sydney’s School of Life and Environmental Sciences.

Byrne noted that the region studied is in a remote, protected area of the reef, far from tourist and industrial influences. However, the reef, “despite its protected status, was not immune to the extreme heat stress that triggered this catastrophic bleaching event,” she said.

Great Barrier Reef under threat

Spanning nearly 133,000 square miles (345,000 square kilometers), the Great Barrier Reef is the world’s largest coral reef system, hosting over 1,500 species of fish and more than 400 types of hard corals. Its significant contribution to Australia’s economy, mainly through tourism, underscores its global importance and the urgency of protective measures.

Nevertheless, recent bleaching episodes illustrate a troubling trend of more frequent and intense heat waves, making it increasingly difficult for corals to recover.

The study’s authors warn that mass bleaching has become “a biennial event,” which “reinforces the need for urgent global action now to adhere to ambitious climate and reduced emissions targets.”

Impact spreads to previously unaffected areas

The latest bleaching event also hit parts of the reef previously spared from mass bleaching, indicating that the crisis is extending into new territories. Even coral species once deemed resilient have exhibited signs of disease and elevated mortality rates.

“Seeing the impacts on a reef that has largely avoided mass bleaching until now is devastating,” said marine scientist and co-author Shawna Foo. “The high rates of mortality and disease, particularly in such a remote and pristine area, highlight the severity of the situation.”

A climate emergency for coral reefs

Severe bleaching events have occurred on the Great Barrier Reef in 1998, 2002, 2016, 2017, 2020, and 2022. The 2022 episode was particularly concerning, as it coincided with a La Niña event – El Niño’s cooler counterpart – raising further doubts about the reef’s resilience to future climate extremes.

The new findings underscore the fragile condition of the reef and the urgent necessity to curb fossil fuel emissions that contribute to global warming. 

With bleaching now seemingly inevitable in many locations, the survival of coral reefs hinges on global efforts to limit temperature rises, offering 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.

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