RESEARCH
Fourth Global Coral Bleaching Event Confirmed as Largest on Record -- 84% of Reefs Affected Since 2023
NOAA, ICRI, and the Global Coral Reef Monitoring Network independently confirm that bleaching-level heat stress hit roughly 84% of the world's coral reef area between January 2023 and mid-2025 -- the fourth global bleaching event on record and by far the most extensive, eclipsing the 68% affected during the 2014-2017 event. The escalation across four events (21% in 1998, 37% in 2010, 68% in 2014-2017, ~84% in 2023-2025) tracks a clear worsening trend, and NOAA had to add three new alert levels to its Bleaching Alert Scale mid-event because the original scale could not capture the mass-mortality risk observed.
Findings
- Bleaching-level heat stress affected roughly 84% of the world's coral reef area across the Pacific, Atlantic, and Indian Ocean basins from January 2023 through mid-2025 -- the most extensive global bleaching event on record.
- The 4th event surpassed the 3rd (2014-2017, 68% of reefs) by about 16 percentage points, continuing a four-event escalation: 21% (1998) to 37% (2010) to 68% (2014-2017) to ~84% (2023-2025).
- Mass coral bleaching was documented in at least 82-83 countries and territories, per ICRI and NOAA.
- NOAA's Coral Reef Watch program added three new levels (3-5) to its Bleaching Alert Scale mid-event to capture heightened mass-mortality risk the original scale could not express.
Delta Engine result
↔ Divergence Detected — Δ 0.1943 (threshold 0.05)
- NOAA Coral Reef Watch T5 96
- ICRI (International Coral Reef Initiative) T0 94
- GCRMN (Global Coral Reef Monitoring Network) T0 90
- Reef Check T0 84
- Earth.org T0 79
Evidence quality
avg 39 · min 25 · max 95 · spread 70