00 · QUICK ORIENTATION
Bleaching is a relationship under stress
- 1When stressed, a coral can lose the microscopic algae living in its tissues, exposing the pale skeleton beneath.
- 2A bleached coral is not necessarily dead, but prolonged stress reduces its chance of recovery.
- 3Heat exposure is cumulative: both intensity and duration matter, and local conditions change how a colony responds.
01 · A TWO-PART ORGANISM
The coral animal hosts a solar-powered partner
A coral polyp is an animal. In many reef-building species, photosynthetic dinoflagellates live inside its cells and exchange products with the host.
The algae supply much of the energy used for growth; the host provides shelter and access to nutrients. Their pigments contribute brown and golden colors.
02 · THE STRESS RESPONSE
Too much heat destabilizes photosynthesis
Unusually warm water can damage the photosynthetic partnership, especially under bright light. Reactive molecules and cellular stress make the alliance costly.
When stressed, a coral can lose the microscopic algae living in its tissues, exposing the pale skeleton beneath.
Healthy symbiosis
Host and algae exchange shelter, nutrients and photosynthetic products.
COLOREDHeat disrupts cells
Photosynthesis and cellular protection become unstable.
EXCESS HEATBleaching
Algae or their pigments are lost and skeleton shows through.
NOT YET DEAD03 · WHAT WHITE MEANS
The skeleton becomes visible through tissue
Coral tissue is thin and relatively transparent. With fewer symbionts or pigments, the calcium-carbonate skeleton beneath dominates what we see.
A bleached coral is not necessarily dead, but prolonged stress reduces its chance of recovery.
Bleached describes a state, not an instant verdict.
04 · THE CLOCK MATTERS
Degrees and weeks combine
A brief temperature spike and several weeks of moderate excess heat are not equivalent, but both can matter. NOAA tracks accumulated thermal stress rather than temperature alone.
Species, symbiont communities, prior exposure, light, water quality and disease all influence the outcome.
When stressed, a coral can lose the microscopic algae living in its tissues, exposing the pale skeleton beneath.
Satellite products estimate exposure across broad areas. Colony-level bleaching and survival require field observation, and tolerance differs among coral–symbiont partnerships.
05 · RECOVERY OR LOSS
Relief can reopen the partnership
If stressful conditions ease, surviving tissue can regain symbionts and color. Recovery takes energy and may leave growth and reproduction impaired.
If heat persists, depleted corals become more vulnerable to disease and mortality. A dead framework can then erode or be overgrown.
Select the stage. The final branch depends on whether stress eases soon enough.
Partnership intact
Symbionts provide energy and help give the colony its color.
HEALTHY06 · FROM POLYP TO PLANET
Satellites watch risk, not every colony
Coral Reef Watch combines sea-surface temperature observations and models to map bleaching-level heat stress across reefs.
Those maps indicate exposure. Field surveys are still needed to determine which corals bleached, survived or recovered.
07 · EVIDENCE TRAIL
From mechanism to global monitoring
NOAA sources separate the biology of one colony from the measured reach of a heat event.
- 01NOAA Ocean Service · What is coral bleaching?OFFICIAL EXPLAINER ↗
Core definition, mechanism and recovery distinction.
- 02NOAA Coral Reef Watch · Satellite monitoringMONITORING SYSTEM ↗
Near-real-time satellite and modeled heat-stress products.
- 03NOAA AOML · The science behind coral bleachingSCIENCE OVERVIEW ↗
How duration, intensity and local thresholds interact.
- 04NOAA NESDIS · Fourth global bleaching event likely ended in 2025DATED UPDATE ↗
Dated status report for the 2023–2025 global event.