The 4th Global Coral Bleaching Event: Ushering in an era of near-annual bleaching

Increasingly frequent marine heatwaves have escalated the prevalence and extent of mass coral bleaching events. Global-scale coral bleaching events (GCBE) can unfold when marine heatwaves impact reefs across all tropical ocean basins within a common period. Here, we define an objective, quantifiable index to compare periods of satellite-derived coral heat stress accumulation globally from 1985–2025 and examine their relationship to confirmed GCBEs. We uncover an uninterrupted and ongoing period of global coral heat stress from 2018 to 2025, affecting an unprecedented 87% of reef areas globally. The average intensity has also been record-setting, with the median global heat stress accumulation on reef areas nearly 50% greater than the previous GCBE record. This is largely driven by general ocean warming, as recent baseline global surface ocean temperatures (2019-2023) are comparable to extreme anomalous conditions observed ~25 years ago. The development of a strong El Niño event in 2023, superimposed upon the already anomalously warm ocean, culminated in the 4th GCBE, the most extensive and intensive on record. Qualitative observations of coral bleaching have been reported from 84 countries spanning all coral-containing ocean basins. Further, an analysis of the extent and magnitude of heat stress suggests that bleaching may have impacted 98 of the 102 countries with coral reefs. GCBE4 (2023–present) marks a period of heat stress that is unprecedented by every investigated metric. However, global-scale coral bleaching-level heat stress has persisted for almost the entirety of the last decade, bringing many reef areas into an era of near-annual bleaching.