How far the fourth global coral bleaching event reached
NOAA’s satellite record puts bleaching-level heat stress across about 84 percent of reef area.
About 84.4 percent of the world's coral reef area experienced enough heat stress to cause bleaching between January 2023 and September 2025, according to NOAA's Coral Reef Watch. Mass bleaching was documented in at least 83 countries and territories over that period.
The estimate describes exposure to heat. Field observations show how corals actually responded, which can vary from one reef and species to another. Corals can lose the algae that help feed them when the water stays unusually warm.
NOAA describes the episode as the largest in its record and says it likely ended in 2025. Its previous high was 68.2 percent of reef area exposed to bleaching-level heat during the 2014–2017 event.
