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The world’s fourth mass coral bleaching event, the fastest and most extensive ever recorded, likely ended in mid-2025. But scientists warn that its conclusion offers little reason for celebration. The event exposed an unprecedented 84% of the world’s coral reef areas to bleaching-level heat stress, affecting reefs across 83 countries and territories in the Pacific, Atlantic and Indian oceans. Researchers warn that the scale of the bleaching points to a future in which extreme coral bleaching events could occur far more frequently as ocean temperatures keep rising.
The fastest and widest coral bleaching event on record
The fourth global coral bleaching event began in early 2023 and was officially confirmed by the US National Oceanic and Atmospheric Administration (NOAA) in April 2024. According to NOAA’s Coral Reef Watch, bleaching-level heat stress affected around 84% of the world’s coral reef area, surpassing the previous global bleaching event between 2014 and 2017, which impacted 68.2% of reefs worldwide.
Scientists determined that the event likely ended after no widespread bleaching was reported during the austral summer between December 2025 and February 2026. The final major episode occurred off the coast of Western Australia in early 2025, which researchers say likely marked the closing phase of the global event.
The scale of the bleaching was unprecedented. NOAA estimates that bleaching was documented in at least 83 countries and territories. At the same time, scientific assessments suggest heat stress severe enough to trigger bleaching may have affected reefs in as many as 98 of the world’s 102 coral reef nations.
Rising ocean temperatures are pushing reefs beyond their limits
Coral bleaching occurs when prolonged marine heat stress causes corals to expel the microscopic algae that provide them with food and colour. Without these algae, corals turn white and become more vulnerable to disease, starvation and death. While corals can recover if temperatures return to normal quickly, repeated bleaching events reduce their chances of survival.
Researchers say the latest event was fuelled by a combination of long-term ocean warming and the strong El Niño conditions that developed in 2023. A recent analysis found that global coral heat stress persisted almost continuously from 2018 to 2025, affecting around 87% of reef areas worldwide and reaching intensities far higher than previous bleaching events.
Derek Manzello, coordinator of NOAA’s Coral Reef Watch, warned that defining the start and end of future global bleaching events may become increasingly difficult as reefs experience heat stress more frequently. Scientists say many reef systems are now approaching conditions where bleaching can occur almost every year.
Nearly a billion people depend on healthy coral reefs
The consequences extend far beyond marine ecosystems. Coral reefs support roughly 25% of all marine species at some stage of their life cycle and provide food, tourism income and coastal protection for communities around the world. Researchers estimate that nearly one billion people depend directly or indirectly on coral reef ecosystems for their livelihoods and well-being.
Scientists are increasingly concerned that some reef systems may be approaching irreversible tipping points. The Global Tipping Points Report 2025 warned that warm-water coral reefs are among the ecosystems most vulnerable to climate change, with repeated bleaching events pushing many reefs toward long-term decline. In parts of the Great Barrier Reef, some areas have already lost more than 70% of their hard coral cover following consecutive marine heatwaves.
Although the fourth global bleaching event may have ended, ocean temperatures remain historically high in many regions. Researchers say the temporary easing of heat stress does not change the broader trend. As greenhouse gas emissions continue to warm the oceans, coral reefs are likely to face increasingly frequent and severe bleaching events, raising concerns about the future of some of the planet’s most biodiverse ecosystems.
References:
https://coralreefwatch.noaa.gov/satellite/research/coral_bleaching_report.php
https://www.nesdis.noaa.gov/news/worlds-fourth-mass-coral-bleaching-event-likely-ended-2025
Banner image: Photo by NEOM on Unsplash
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