Coral Bleaching Awareness Month – Summary of the Fourth Global Coral Bleaching Event
Since August 2018, the world’s coral reefs have been exposed to chronically high levels of temperature stress that has caused in widespread coral bleaching. This Global Coral Bleaching Event (GCBE), is the fourth recorded to date, and it is the biggest in terms of its geographical extent, duration and intensity – with 96% of all coral reef-dependent countries affected.
Within this article, we are going to explore the results of a recent publication: The 4th Global Coral Bleaching Event: Unprecedented, unbounded, unrelenting (Spady et al. in press).
Key Points
Global Coral Bleaching Events (GCBEs) occur when heat stress affects reefs across all three major tropical ocean basins for an extended period within a 12-month window.
Four GCBEs have been recorded since 1997, each one more widespread, prolonged, and severe than the last.
GCBEs are lasting longer and becming more frequent, leaving reefs with less time to recover in between episodes.
GCBE4 (2018 – present) is the longest, (6.8 years) and most geographically extensive bleaching event ever recorded, impacting over 78% of global reef area.
What is a Global Coral Bleaching Event?
For a bleaching event to be classified as a GCBE it must meet the following criteria:
Widespread regional stress – each of the three major tropical ocean basins (the Atlantic, Indian, and Pacific) must simultaneously experience bleaching-level heat stress (≥4°C-weeks) across at least 12% of their reef areas.
Sustained duration – these conditions must persist for at least 217 consecutive days (approximately 31 weeks).
Global extent – during that same period, the total global reef area exposed to bleaching-level heat stress (≥4°C-weeks) must reach at least 20%.
These events signal that ocean temperatures have risen to a level capable of triggering mass coral bleaching on a global scale.
Coral bleaching occurs when corals expel the symbiotic algae (zooxanthellae) that provides them with 90-95% of their energy and their colour. The most common trigger for bleaching is prolonged preriods of seawater temperatures above the summer maximum, which is measured in degree heating weeks (DHW).
To dive deeper into the science behind coral bleaching and for further information on DHW, check out our blog post from last week:
🔗Coral bleaching Awareness Month – Understanding the Crisis Beneath the Waves
Previous Global Coral Bleaching Events
The first recorded GCBE started in 1997, marking the beginning of a new era of climate-driven reef stress. Since then, a further three events have been recorded, each more widespread, prolonged, and devastating than the last.
Figure 1: Daily global sea surface temperature anomalies & recorded GCBEs between 1995 - June 1st 2025 Sources: www.climatecentral.org; Spady et al., in press.
From the above timeline, we can clearly see a troubling trend – GCBEs are becoming longer and more frequent, with shorter recovery windows between events. This pattern suggests that coral reefs are now under near-constant thermal stress, leaving little opportunity for recovery and regeneration.
Figure 2: Maximum percentage of global and regional coral reef area experiencing bleaching level heat stress (≥4°C-weeks), within a 365-day period of each GCBE. Source: Spady et al., in press.
Overall, this data shows that global bleaching events are no longer rare disturbances - they have become a recurring and intensifying feature of our changing climate, with profound implications for reef resilience and long-term ecosystem stability.
What is Significant About The Fourth Global Bleaching Event?
The fourth GCBE is unlike any previous event in both scale and persistence. It marks a turning point in our understanding of these events; they are no longer rare crisis but a sustained pressure reshaping reef ecosystems worldwide.
Here is what makes GCBE4 so significant:
Unprecedented geographical reach – satellite data indicates that 97 out of 101 coral reef-containing counties are likely to have experienced bleaching; 83 of these have been confirmed with verified field observations.
Record-breaking duration – Lasting 6.8 years (as of 1st June 2025), GCBE4 is the longest GCBE on record, nearly doubling the length of the previous event (GCBE3) which lasted 3.5 years.
Compressed recovery window – satellite data indicates that GCBE3 and GCBE4 were separated by only 175 days, indicating that corals had little to no time to recover before being re-exposed to stress.
Higher cumulative stress – GCBE has produced the highest recorded global bleaching stress extent (see Figure 2).
Refugia areas compromised – mass coral bleaching was observed for the first time in areas that were previously considered ‘thermal refugia’ .
What Does This Mean for Coral Reefs?
As the ecological baseline for corals shift, the impacts are becoming increasingly severe:
Recovery windows are shrinking – without time to regrow and reproduce, coral survival rates drop.
Recruitment is disrupted – repeated stress events disrupt coral spawning and larval settlement, slowing population growth and turnover.
Species composition is changing - as more heat-sensitive species decline, reef composition may shift towards tolerant species, reducing habitat complexity, biodiversity and resilience.
Increased vulnerability to local stressors – bleached, starved corals are weakened and more susceptible to disease, overgrowth, sedimentation, water quality changes and physical damage.
The Global Tipping Point Report 2025 concluded that coral reefs have already crossed a critical threshold. For more information on what this means, check out our previous article:
🔗 Coral Reefs in Crisis: Insights from the Global Tipping Points Report 2025
What Does This Mean for People?
GCBE4 is not just an ecological crisis – it’s a human one. Coral reefs are essential ecosystems that support the lives and livelihoods of over 500 million people.
Food security – coral reefs support fisheries that feed millions
Coastal protection – healthy reefs buffer shorelines from storms and erosion
Tourism-based income – coral reefs generate billions in tourism revenue and jobs
Cultural significance – for many indigenous and coastal communities, reefs are central to cultural identity
Learn More About Coral Bleaching
This week, we have focused on the scale and significance of the fourth GCBE, bringing to light our current situation.
Next week, we’ll shift to practical action by discussing what can be done to support reef recovery, reduce local stressors and build resilience in the face of sustained global pressure.
References
Spady, B.L., Skirving, W.J., Jacqueline, L., Geiger, E.F., Liu, G., Hoegh-Guldberg, O., Norrie, A., Heron, S.F., Pomeroy, M., Kolodziej, G. and Manzello, D.P., in press. The 4th Global Coral Bleaching Event: Unprecedented, unbounded, unrelenting.
