Key figures
- About 84% of global reef area exposed to bleaching-level heat stress since January 2023 (NOAA)
- Mass bleaching documented in at least 83 countries and territories
- The previous record, in 2014–2017, affected about 68% of reef area
- Reefs support at least a quarter of all marine species
Coral reefs cover less than one percent of the ocean floor, yet they are among the richest ecosystems on the planet. They shelter at least a quarter of all marine species, from tiny shrimp to sharks and sea turtles. They act as natural breakwaters, absorbing the energy of waves and protecting coastal towns from storms. They support fisheries that feed hundreds of millions of people, draw tourists who sustain entire island economies, and harbour compounds used to develop new medicines. Researchers estimate the benefits reefs provide to society at close to US$10 trillion a year.
Right now, those reefs are enduring the most severe and widespread heat stress ever recorded.
The challenge: the fourth global bleaching event
The fourth global coral bleaching event began in early 2023, driven by record ocean temperatures linked to climate change and amplified by a strong El Niño. The US National Oceanic and Atmospheric Administration (NOAA) officially declared it a global event in April 2024. By 2025, according to NOAA’s Coral Reef Watch program, bleaching-level heat stress had affected about 84% of the world’s coral reef area, and mass bleaching had been documented in the waters of at least 83 countries and territories. The previous record, set during the third global event between 2014 and 2017, was about 68%. The current event is not only larger but more intense: the median heat stress accumulated on reefs is nearly 50% greater than the previous record, and scientists see no clear sign of when it will end.
To understand why this matters, it helps to understand how corals live. A coral is a partnership between a tiny animal related to jellyfish, which builds the hard limestone skeleton, and microscopic algae that live inside its tissues. The algae use sunlight to produce food through photosynthesis and give the coral most of its energy — and its vivid colours. When the water becomes too warm for too long, this partnership breaks down. The coral expels the algae, revealing its white skeleton. That is bleaching.
A bleached coral is not yet dead, but it is starving. If temperatures fall quickly enough, the algae can return and the coral can recover over months or years. If the heat persists, the coral dies and is soon covered by seaweed. A study published in Nature Communications in February 2026 calculated, for the first time, the global toll of a bleaching event: during 2014–2017, more than half of the world’s reefs suffered significant bleaching and about 15% experienced significant coral death. The current event is more severe.
Individual reef systems tell the same story. Australia’s Great Barrier Reef, the largest reef system on Earth, suffered its sixth mass bleaching since 2016 in 2025 — the second time it has bleached in consecutive years. Reefs in Florida, the Caribbean, the Red Sea, the western Indian Ocean and Southeast Asia have all been hit hard. Projections suggest that, even under conservative scenarios, most reefs in the world could experience severe bleaching every year by the middle of the century, leaving no time for recovery between events.
Heat is not the only threat. Reefs are also weakened by pollution from sewage and farm fertilisers, sediment from coastal construction and deforestation, destructive fishing practices and overfishing of the grazing fish that keep seaweed under control. As the ocean absorbs carbon dioxide, it also becomes more acidic, making it harder for corals to build their skeletons.
What can be done
Scientists agree that the only long-term solution is to limit ocean warming by rapidly reducing greenhouse gas emissions. Keeping global warming as close as possible to 1.5 °C gives a significant share of reefs a chance of survival; at 2 °C or more, the outlook for most warm-water reefs becomes bleak.
But researchers are equally clear that local action can dramatically improve a reef’s chances of surviving the heat that is already coming. Healthy reefs recover from bleaching far better than stressed ones.
Reducing local pressure is the first step. Treating sewage, reducing fertiliser runoff from farms and controlling coastal erosion keep reef waters clean and clear. Sustainable fisheries management protects the parrotfish and other grazers that clean algae off coral surfaces, making space for young corals to settle. On the Mesoamerican Reef, which stretches for about 1,000 kilometres along the coasts of Mexico, Belize, Guatemala and Honduras, bleaching affected around 40% of the reef in 2024. Yet some areas recovered well, and scientists attribute part of that resilience to better fisheries management that had allowed fish populations to rebound.
Marine protected areas are another powerful tool, as long as they are properly enforced and designed together with local fishing communities. Protection gives reefs space to rebuild and helps nearby fisheries through the ‘spillover’ of fish from protected zones. Scientists also recommend prioritising the protection of deeper reefs and reefs in naturally cooler areas, which may act as refuges — places where corals survive heatwaves and can later help repopulate damaged areas.
Active restoration is growing quickly. Around the world, teams are growing coral fragments in underwater nurseries and replanting them on degraded reefs. Others are breeding corals that tolerate heat better, collecting coral spawn during mass spawning events to raise millions of larvae, or testing temporary shading and cooling techniques for small, high-value sites. These methods cannot be applied to the whole ocean, and they are no substitute for emission cuts, but they can protect important sites, maintain genetic diversity and buy time.
Better monitoring helps managers act quickly. Satellite-based heat stress alerts from NOAA’s Coral Reef Watch give reef managers weeks of warning, allowing them to temporarily close sites to tourism, reduce other stressors or move coral nurseries to cooler water.
What you can do
Visitors to reef destinations can choose dive and snorkel operators that follow responsible practices, avoid touching or standing on coral, and support marine parks through entry fees. Choosing reef-safe sunscreen, eating sustainably caught seafood and taking part in citizen-science programmes such as Reef Check or CoralWatch all contribute. Above all, supporting climate action at home helps reefs thousands of kilometres away.
Coral reefs have existed in various forms for hundreds of millions of years and survived dramatic changes in the past. But they have never faced warming this rapid. Whether today’s reefs survive this century depends largely on decisions made in this decade.
Sources
NOAA Coral Reef Watch; International Coral Reef Initiative (April 2025); Eakin et al., Nature Communications (February 2026); Smithsonian Tropical Research Institute (March 2026); Earth.org (2026); ScienceAlert (January 2026).
