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Coral bleaching happens when heat stress disrupts the partnership between reef-building corals and the microscopic algae living in their tissues. Corals may lose or expel many of those algae, making their tissues look pale or white and depriving them of an important food source. Bleaching is a sign of stress, not proof that a coral is dead; prolonged or severe heat raises the risk of mortality.
Why corals bleach
Reef-building corals live in partnership with microscopic algae in their tissues. The algae contribute color and provide food. NOAA explains that prolonged extreme heat can break down this beneficial relationship; when many algae and their pigments are lost, the coral’s tissue becomes translucent and the pale skeleton beneath shows through. NOAA describes coral bleaching and its causes, while NOAA’s overview of threats to coral discusses how heat can disrupt the partnership.
The white appearance is therefore a visible sign of stress, not a diagnosis of death. A bleached coral has lost an important source of nourishment and may be more vulnerable. Some corals can recover if conditions improve and they regain their symbiotic algae; prolonged or intense stress can instead lead to death. The outcome varies among corals and locations.
How heat stress triggers bleaching
There is no single temperature threshold that applies to every coral species, depth, and region. NOAA/AOML describes two broad patterns of thermal exposure that can trigger bleaching: brief periods of very hot water lasting several days, and moderately elevated temperatures that persist for weeks. The local coral community and its history matter, so long-term observations near a reef help identify conditions associated with bleaching there.
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This variation is one reason a heat alert should be read as a risk signal rather than a prediction that every coral in a mapped area will bleach or die. Satellite sea-surface products summarize conditions across mapped pixels; they do not show that every coral at a site experiences identical temperatures or biological outcomes.
How NOAA measures accumulated heat stress
NOAA Coral Reef Watch uses Degree Heating Weeks (DHW) to summarize accumulated thermal stress. The measure adds Coral Bleaching HotSpots—temperatures more than 1°C above the local bleaching threshold—over a rolling 12-week period and reports the total in °C-weeks. It captures both the intensity and duration of heat exposure; it is an indicator of stress, not a direct count of bleached coral colonies. See NOAA Coral Reef Watch’s methodology and its 5-km methodology and product information.
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| DHW level | NOAA Coral Reef Watch interpretation |
|---|---|
| Above 4°C-weeks | Significant coral bleaching has been shown to occur at these levels. |
| Above 8°C-weeks | Associated with severe, widespread bleaching and significant mortality; the 5-km methodology says reef-wide bleaching with mortality among heat-sensitive corals can be expected by this level. |
These are NOAA monitoring thresholds and risk expectations, not deterministic rules for every reef. NOAA Coral Reef Watch’s 50-km methodology states: “Mass coral bleaching has been shown to be caused by prolonged periods of thermal stress.” The local community, exposure pattern, and other conditions influence what happens on a particular reef.
What the latest global bleaching figures mean
In an update dated June 2, 2026, NOAA said the fourth global mass coral bleaching event likely ended in 2025. For the defined period from January 1, 2023, through September 30, 2025, NOAA reported:
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- Bleaching-level heat stress affected approximately 84.4% of the world’s coral reef area.
- Mass bleaching was documented in at least 83 countries and territories.
- For comparison, 68.2% of global reef area experienced bleaching-level heat stress during the 2014–2017 third global bleaching event.
These figures describe the event period through September 30, 2025; they are not a live reading of reef conditions today. NOAA’s June 2, 2026 global bleaching update provides the event summary.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to interpret bleaching risk at a particular reef
When comparing locations, consider the accumulated heat stress and the period over which it occurred, alongside local temperature thresholds and long-term observations. Also consider the coral community present: heat tolerance varies. A satellite-derived DHW value helps describe thermal exposure, but it cannot by itself establish how many colonies are bleached or whether a specific coral will survive.
Bleaching across many colonies can reduce living coral and affect the wider reef ecosystem. But outcomes are not uniform: bleaching does not mean every coral dies, and a regional heat-stress indicator does not describe the condition of each coral individually. NOAA’s historical reporting on marine heatwaves and coral impacts offers further context in its report on marine heatwaves and corals.
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