The world’s fourth global coral bleaching event likely ended in mid-2025 after subjecting 84% of coral reefs to bleaching-level heat stress, according to satellite analysis by NOAA’s Coral Reef Watch.
Between early 2023 and mid-2025, mass bleaching was documented in at least 83 countries and territories across the Pacific, Atlantic and Indian oceans. The event concluded after severe bleaching struck Western Australia in early 2025, following which global heat stress declined and only isolated bleaching reports surfaced.
“We needed to confirm that no widespread, large-scale bleaching was reported anywhere during the austral summer which ran from December 2025 through February 2026, before we were confident the event had ended,” said Derek Manzello, coordinator of NOAA’s Coral Reef Watch.
The determination required careful tracking. Manzello noted that reefs now bleach on a near-annual basis, making it increasingly difficult to define when global events begin and end. Moving forwards, NOAA will rely on field bleaching observations rather than satellite data alone to determine whether global events are occurring.
El Niño threat.
The respite may prove short-lived. NOAA’s four-month coral bleaching outlook shows high risk across much of the north Pacific Ocean, including Hawai’i, as well as Florida and the Caribbean later this summer, driven by an expected El Niño emergence. Since 1998, every strong El Niño has coincided with global coral bleaching, with heat stress becoming more widespread and severe with each event. The first and second global bleaching events occurred in 1998 and 2010; the third spanned three years, from 2014 to 2017.
Despite the fourth event’s massive scale, not all coral areas bleached when exposed to high ocean temperatures. Scientists are investigating these locations to understand what factors contributed to heat resistance. “NOAA and its partners are studying multiple aspects of heat tolerance in corals,” said Jennifer Koss, director of NOAA’s Coral Reef Conservation Program. “This will help us better understand resilience in corals and improve coral restoration strategies and techniques across the nation.”
Sea surface temperatures remain higher than 25 to 30 years ago, when the first global bleaching event occurred. Manzello emphasised that thermal stress is now pervasive on coral reefs, making frequent monitoring vital to understand the biological and physical factors associated with bleaching resilience from the organism to ecosystem level.
Source: NOAA National Centers for Environmental Information.
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