Global coral heat stress reached record levels from 2018 to 2025

What the study found: Global coral heat stress has been uninterrupted from 2018 to 2025, affecting an unprecedented 87% of reef areas worldwide. The abstract describes this period as the 4th global coral bleaching event (a globally widespread episode of coral bleaching linked to marine heat stress) and says it is the most extensive and intensive on record.
Why the authors say this matters: The authors conclude that global-scale coral bleaching-level heat stress has persisted for almost the entire last decade, bringing many reef areas into an era of near-annual bleaching. They also note that the findings suggest bleaching may have affected nearly all countries with coral reefs.
What the researchers tested: The researchers created an objective, quantifiable index to compare satellite-derived coral heat stress accumulation globally from 1985 to 2025. They then examined how those heat-stress periods related to confirmed global coral bleaching events.
What worked and what didn't: The analysis found a record-setting median global heat stress accumulation on reef areas, nearly 50% greater than the previous global coral bleaching event record. The abstract says this was largely driven by general ocean warming, with a strong El Niño event in 2023 adding to already anomalously warm ocean conditions and culminating in GCBE4.
What to keep in mind: The abstract does not provide detailed limitations of the index or the analysis. It also refers to qualitative bleaching reports from 84 countries and an analysis suggesting possible impact in 98 of 102 coral-reef countries, but it does not give country-level confirmation for every location.

Key points

• Global coral heat stress was uninterrupted from 2018 to 2025.
• The affected area reached 87% of reef areas globally.
• The authors describe GCBE4 as the most extensive and intensive global coral bleaching event on record.
• Median global heat stress accumulation was nearly 50% higher than the previous record.
• The abstract says general ocean warming and a 2023 El Niño event drove the record heat stress.
• Qualitative bleaching reports came from 84 countries, and heat-stress analysis suggested possible impact in 98 of 102 coral-reef countries.