The fourth global mass coral bleaching event, spanning 2023–2025, hit a larger share of the world’s reefs than any bleaching event ever recorded — a stark escalation from the first global event in 1998.
Coral reefs cover less than 1% of the ocean floor but support an estimated quarter of all marine species at some point in their life cycle, making them among the most biodiversity-dense ecosystems on the planet. They are also among the most directly threatened by ocean warming.
From January 2023 through September 2025, bleaching-level heat stress affected approximately 84.4% of the world's coral reef area, with mass bleaching documented in at least 83 countries and territories — the fourth global mass bleaching event on record, and the most extensive ever measured.
Source: NOAA Coral Reef Watch; International Coral Reef Initiative, 2025The Great Barrier Reef bleached for a sixth time since 2016 in 2025, following a fifth event in 2024 — only the second time on record it has bleached in consecutive years. AIMS's 2024/25 condition summary recorded the largest annual coral cover decline in two of the Reef's three regions in 39 years of monitoring, with some individual reefs down as much as 70.8%. Researchers describe 2025 as the most spatially extensive bleaching event the Reef has recorded since AIMS's monitoring began in 1986, and it was also the first time the Reef and Western Australia's Ningaloo Reef bleached simultaneously — Ningaloo alone lost more than 60% of the corals scientists were tracking there over a single season. A fuller status report is available in our Field Notes: Great Barrier Reef.
Not every recent reef story is one of loss. In July 2026, a research team led by Gérard Zinzindohoué of the Benin Fisheries and Oceanological Research Institute returned to a reef off the coast of Benin, in the Gulf of Guinea, that had been presumed dead based on surveys roughly 60 years earlier.
Using high-resolution sonar, an underwater drone and the National Geographic Exploration Technology Lab's deep-sea camera system, researchers found the reef alive and active: eight coral types and eight fish species were documented sheltering and feeding around the site. The team's analysis suggests it is a mesophotic coral ecosystem — a deep-water, low-light-adapted community that often forms scattered patches rather than a continuous barrier reef — and describes it as the first confirmed living mesophotic coral ecosystem documented on the Gulf of Guinea's continental shelf.
Source: Zinzindohoué et al., Frontiers in Marine Science, 2026The finding is a useful reminder that "presumed dead" and "confirmed dead" are not the same thing, particularly for deep or hard-to-survey reef systems: a 1960s-era survey using the technology available at the time simply may not have been capable of detecting a reef adapted to low-light, mesophotic depths in the first place.
Satellite-based heat-stress monitoring provides strong global coverage of bleaching risk, but on-the-ground verification of actual coral mortality (as opposed to bleaching, from which some corals recover) is more limited and concentrated in well-studied reef systems like the Great Barrier Reef and the Florida Keys.