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Ecosystem Profile

Coral Reefs

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.

Published May 2026 Last reviewed July 2026 Evidence level Strong Reading time 6 min

Overview

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.

Established fact

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, 2025

Escalating Scale

1998First global bleaching event — 21% of reefs affected by bleaching-level heat stress.
2010Second global event — 37% of reefs affected.
2014–17Third global event — 68% of reefs affected.
2023–25Fourth global event — 84.4% of reefs affected, the most extensive on record; NOAA reports the event likely concluded by mid-2025 as heat stress declined.

Regional Impact: Great Barrier Reef

The Great Barrier Reef experienced its fifth mass bleaching event since 2016 in 2024, with regional coral cover declines of 14–30% compared to 2024 levels, and some individual reefs recording declines of up to 70.8%, according to the Australian Institute of Marine Science's 2024/25 condition summary.

Pressures

Marine heatwavesSustained above-normal ocean temperatures cause corals to expel the symbiotic algae they depend on for energy, a process called bleaching; repeated or prolonged bleaching events can cause mass coral mortality rather than recovery.
Ocean acidificationRising atmospheric CO2 is gradually lowering ocean pH, making it more difficult for corals to build and maintain their calcium carbonate skeletons.
Local pollution and overfishingNutrient runoff and sedimentation from coastal development, along with overfishing of herbivorous reef fish, can reduce reef resilience and slow recovery between bleaching events.

Uncertainty & Evidence Gaps

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.