Definition and Importance:
Coral bleaching is a stress response in corals, where they expel the symbiotic algae (zooxanthellae) that live in their tissues, causing them to turn white. This occurs due to  various stressors, with elevated temperature being the cause of widespread mass-bleaching events. Bleached corals are more vulnerable to disease, have reduced growth rates, and can die if stressful conditions persist. Monitoring coral bleaching helps gauge the impact of environmental changes and the resilience of reef ecosystems.

Limiting global warming to 1.5^oC, requires greenhouse gas emissions be cut 4% by 2030. Even if all current national commitments are met by 2030, we will have exceeded 1.5^oC. Coral reef survival is debatable at 1.5^o warming—but is highly unlikely at the higher levels we are quickly moving towards. Dramatic changes are required at national and individual levels. Mass coral bleaching events occur when elevated sea temperatures cause corals to expel the symbiotic algae in their tissues, resulting in the loss of their color and energy source. This makes corals more vulnerable to disease and death, threatening marine biodiversity and disrupting ecosystems.

Reefs in and around the Caribbean are experiencing high levels of heat stress. Bleaching events have become more frequent, intense, and prolonged over the last decade, leading to widespread damage and reduced resilience of coral populations.  We have experienced high heat stress in the Caribbean and particularly in the MAR since 2015. The 2023 event was much more severe causingconsiderable coral mortality, particularly in some of our most iconic reefs. 

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How Do We Measure it?
Coral bleaching is generally assessed during benthic surveys or BleachWatch initiatives using “bar drop”, coral belt-transects, or photoquadrants. Divers examine coral colonies (roving or following a transect), recording the extent of bleaching observed in each colony. Data are typically categorized by species and the severity of bleaching (e.g., pale, partly bleached, fully bleached, partial mortality). These methods help document the prevalence and intensity of bleaching events, providing valuable data for understanding the impacts of temperature stress on reef health.