On June 2, 2026, NOAA announced the likely end of the fourth global coral bleaching event. Between early 2023 and mid-2025, 84% of the world’s reef surface experienced heat intense enough to cause bleaching, affecting 83 countries and territories.
More than four out of five reefs.
This page previously mentioned “4 threats” without date or source. It has been revised around bleaching, with the 2026 assessments from NOAA and the Global Coral Reef Monitoring Network (GCRMN). For polyp biology, our complete coral guide takes over.
What is the fourth global coral bleaching event?
It is the marine heatwave that caused reefs to bleach in the Pacific, Atlantic, and Indian Oceans between early 2023 and mid-2025. NOAA’s Coral Reef Watch program, which monitors reef thermal stress via satellite, recorded 84% of the global reef surface affected, across 83 countries and territories.
The previous three events occurred in 1998, 2010, and from 2014 to 2017, and this one is the most extensive ever measured by the program.
| Global Event | Period | Key Takeaways |
|---|---|---|
| 1st | 1998 | Significant decline in global coral cover (GCRMN, 2026) |
| 2nd | 2010 | Further global decline in cover (GCRMN, 2026) |
| 3rd | 2014 to 2017 | Three years of stress, then a relative recovery of about 6% between 2017 and 2019 (GCRMN, 2026) |
| 4th | 2023 to 2025 | 84% of reef surface affected, 83 countries and territories (NOAA, 2026) |
But NOAA emphasizes something other than surface area. According to Derek Manzello, coordinator of Coral Reef Watch, reefs are entering an era where they will bleach almost every year, making it difficult to say when a global event begins and ends. There is little calm between heatwaves, while coral needs quiet years to recover. The frequency matters more than the record. A record impresses, but an almost annual pace deprives colonies of the time they need to grow.
At the scale of a single reef, the toll is even heavier. The Great Barrier Reef has experienced six mass bleaching events since 2016 according to the Australian institute AIMS, as detailed in our update on the state of the Great Barrier Reef in 2026.
Why does coral bleach when the water heats up?
Because heat breaks the association between the polyp and its microalgae, the zooxanthellae. When water stays above its usual maximum for several weeks, the coral expels these algae that gave it its color and most of its energy. Its white skeleton then appears through tissues that have become transparent.
Bleached coral is still alive, for a time.
It draws on its reserves. If the temperature drops quickly enough, it can recover algae and regain its colors in a few weeks or months. If the stress continues, it starves to death, then filamentous algae cover the skeleton. However, the duration of heat matters as much as its peak. Coral Reef Watch calculates a cumulative thermal stress, which adds week by week the heat above the reef’s usual maximum, rather than a simple temperature recorded on a given day.
A long and slightly too warm summer can therefore cause more damage than a brief peak.
The previous version of this page claimed that coral does not survive more than two or three months at 31 °C. No source was found for this single threshold. Each reef has its own, and corals in the Gulf of Aqaba, for example, tolerate temperatures that would bleach other populations, as explained in our article on Red Sea corals facing warming.
How much coral have the world’s reefs lost?
According to the GCRMN report published on August 31, 2026, the average hard coral cover dropped from 30.2% during 1980 to 2009 to 27.3% during 2020 to 2024, a relative decline of 9.5%. Between 2023 and 2024 alone, it lost 8.9% relatively. This is the largest annual decline ever recorded.
The GCRMN is the global reef monitoring network, linked to the International Coral Reef Initiative (ICRI). Its 2025 report is based on 21.1 million observations from 36,886 sites, with about 600 partners in more than 120 countries and territories. No other assessment is broader. Yet it stops at 2024. The authors warn that the full impact of the fourth episode is not yet included.
For the year 2024 alone, the average global coverage of hard coral fell to 25.8%, according to the same estimate.
Macroalgae, on the other hand, are gaining ground. Their average coverage on reefs has increased by 44.1% since 1980, according to the same report.
And there is some good news, to be read with caution. After the third episode, global coverage recovered by about 6% between 2017 and 2019, which proves that reefs can regenerate when given a few years of respite. Except that since 2010, episodes have been occurring every four to six years, compared to about one per decade previously. The respite is shrinking.
What other threats worsen coral bleaching?
Warming remains the primary cause of global decline, according to the GCRMN. Other, more local pressures, however, reduce the corals’ margin against heat: pollution from land, ocean acidification, outbreaks of crown-of-thorns starfish, and diseases. An already weakened reef recovers less well after an episode.
The IUCN has measured the result of this stacking of threats.
In November 2024, 44% of reef-building coral species were threatened with extinction according to its Red List, with climate change listed as the leading cause.
Pollution and sunscreen
Sediments, fertilizers, and wastewater cloud the water and feed algae that compete with coral on the seabed. Some sunscreens, such as oxybenzone, are toxic to coral larvae in laboratory settings. Their effect on an open reef remains debated. The previous version mentioned “thousands of tons” of toxic products dispersed with each swim, a figure without source that has been removed. We provide an update in our article on sunscreen and marine environments.
Ocean acidification
However, the chemistry of the water is also changing. The ocean absorbs some of the CO2 we emit, its pH decreases, and the carbonate ions that coral uses to build its skeleton become scarcer. The skeleton then grows more slowly and breaks more easily.
Acanthaster and diseases
The crown-of-thorns starfish (Acanthaster) devours polyps. On the Great Barrier Reef, the marine park authority ranks it among the three main causes of coral decline, along with cyclones and bleaching. It is native to the reef, contrary to what the previous version suggested, and the problem comes from its outbreaks, the causes of which remain debated. Our article on vinegar control of Acanthaster details what can be done.
What do we lose when a reef bleaches and then dies?
First, habitat is lost. A dead reef becomes covered with algae, its skeleton erodes, and the fish that found shelter and food there become rarer. Coastal communities lose with it part of their fishing, tourism income, and a natural barrier that breaks waves before the beach.
This last point is hard to see from a boat.
A living reef grows and repairs its breaches, whereas a dead reef only wears away, and over time its structure flattens and lets more wave energy pass to the shore, at the very moment when sea levels are rising. Thus, the damage from bleaching is also visible on land. It occurs years after the heat wave, when no one makes the connection anymore.
The previous version quantified reef tourism in “billions of euros” and spoke of forced migrations, without any source. We have kept the mechanisms and removed these claims. How many people depend on a given reef? Impossible to decide here. Estimates exist, but they vary too much from one coast to another to be generalized here.
What can you do as a diver or traveler?
Your direct impact on the climate remains modest, but you can limit local damage to the reefs you visit. Don’t touch anything, work on your buoyancy, limit sunscreen use with a rash guard, and check the recent state of the reef before booking. Coral stressed by heat tolerates even less a fin kick.
A reef photographed in 2022 may have bleached by 2024.
Booking platforms and operator pages rarely display the date of their images, and nothing distinguishes a reef photographed before 2023 from a reef as it is today. Ask the center for recent and dated observations. A serious center generally knows whether its sites bleached the previous summer, and which ones have regained color since. In short, a turquoise photo dates nothing. And a bleached reef remains a diving spot, just less colorful (we prefer you know this before departure rather than after putting your head underwater).
For basic actions, reread why you should never touch marine life. The cost for you is minimal: a few questions before paying, a lycra suit costing a few dozen euros, a dive dedicated to buoyancy. For a colony that took centuries to grow, that’s a bit of margin saved.
FAQ
How many reefs bleached during the fourth global event?
According to NOAA, 84% of the world’s coral reef surface experienced thermal stress strong enough to cause bleaching between early 2023 and mid-2025. The phenomenon was documented in 83 countries and territories.
Can a bleached coral regain its colors?
Yes, if the water cools quickly enough. The coral can then recover symbiotic algae and recolor in a few weeks or months. If the heat lasts too long, it dies and becomes covered with algae.
Is the fourth global bleaching event over?
NOAA announced in June 2026 that it probably ended in 2025. However, its Coral Reef Watch coordinator believes reefs will now bleach almost every year, making it difficult to pinpoint the end of an event.
What is the main threat to corals?
Marine heatwaves linked to climate change, according to GCRMN and IUCN. Pollution, acidification, outbreaks of crown-of-thorns starfish, and diseases then reduce the reefs’ ability to recover between events.
Sources
- World’s Fourth Mass Coral Bleaching Event Likely Ended in 2025 (NOAA NESDIS, 2026)
- Status of Coral Reefs of the World: 2025, Executive Summary (GCRMN / ICRI, 2026)
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