What is coral bleaching?
Coral is an animal. Each coral head is built from thousands of tiny polyps that live symbiotically with microscopic algae called zooxanthellae. These algae live inside the coral’s tissue, convert sunlight into food, and give coral its colour.
When the ocean gets too warm — even one degree above the seasonal maximum, sustained over weeks — the relationship breaks down. Coral expels its algae. The tissue goes transparent, the white skeleton shows through. This is bleaching. The coral is not dead, but it is starving. If temperatures drop in time, the algae can return. If they don’t, the coral dies.
Mass bleaching events were rare before the 1980s. They are now routine. Four global events have been confirmed: 1998, 2010, 2016, and 2024 — the largest on record, still ongoing.
Why reefs matter
Reefs cover less than 1% of the ocean floor and support roughly a quarter of all marine species. They are nurseries and feeding grounds for fish populations that billions of people depend on for food. A healthy reef absorbs up to 97% of wave energy, protecting coastlines from storm surge. They generate hundreds of billions in fishing and tourism each year.
When a reef dies the structure collapses over years. The fish leave. The coastlines erode. Recovery, if it happens at all, takes decades.
What can be done
The primary driver is ocean warming from climate change. Reducing carbon emissions is the only way to slow the trajectory. Locally, reefs survive better where fishing pressure is managed, coastal runoff controlled, and marine protected areas enforced. Reefs in well-managed areas recover measurably faster after bleaching.
Coral restoration programs — growing heat-tolerant fragments in nurseries and transplanting them — are expanding in Florida, the Caribbean, Australia, and the Indo-Pacific. Genetic research on naturally resistant populations offers longer-term possibilities. None of it works without addressing warming at its source.
Alert scale
No StressTemperatures within normal range
WatchApproaching bleaching threshold
WarningHeat stress active — bleaching likely
Alert 1Significant bleaching — mortality possible
Alert 2Mass bleaching — widespread mortality expected
Measurements
SSTSea surface temperature, recorded via satellite in °C or °F
AnomalyDegrees above the historical average for that time of year. +1°C sustained over weeks causes bleaching.
DHWDegree Heating Weeks — accumulated thermal stress over a rolling 12-week window. Above 4 = bleaching. Above 8 = mass mortality.
Health0–100 index combining current DHW, anomaly, and the station’s bleaching history.
ConnectivityReefs replenish by releasing larvae that drift on currents. Curved lines on the map trace these dispersal corridors.
Global bleaching events
1998First global event. El Niño drove temperatures 4°C above normal across the Indian Ocean. An estimated 16% of the world’s coral died.
2010Second global event. Severe bleaching across the Indian Ocean and Coral Triangle.
2016Third global event. Over 50% of shallow coral in the northern Great Barrier Reef died. Bleaching reached reefs that had never been affected before.
2024Fourth global event — the largest confirmed on record. Bleaching across every ocean basin simultaneously. Still ongoing.
Reading the map
Each circle marks a NOAA monitoring station. Size encodes the current bleaching alert level — all dots are white. A small outline circle means no stress. The largest filled circles indicate Alert Level 2, where mass coral mortality is expected.
Thin curved lines trace larval dispersal corridors. When a source reef bleaches severely, it disrupts the supply of new coral recruits to reefs downstream. Drag to pan. Scrub the timeline to move through 1985 to present. Switch to globe view to rotate.
Data
Thank you to the scientists, field researchers, and data teams at these institutions.
Global Coral Reef Monitoring Network (GCRMN) —
gcrmn.netOcean Biodiversity Information System (OBIS) —
obis.org
Data is continuously updated as new satellite readings and field surveys are published. Conditions shown may change daily.
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