Coral reefs are among the most productive ecosystems on Earth, built slowly by animals that resemble plants. Each coral polyp hosts microscopic algae that convert sunlight into energy. When ocean temperatures rise, this partnership breaks down. The coral expels the algae, loses its color, and turns white — a process known as bleaching. The coral is still alive, but weakened. If stressful conditions persist, many die.
Mass bleaching events were once rare. Over the past four decades they have become increasingly frequent and severe, driven by warming seas. A rise of just 1–2°C above typical summer temperatures can trigger bleaching across entire regions.
A global analysis of the Third Global Coral Bleaching Event (2014–2017) provides a stark benchmark. Marine heatwaves affected reefs worldwide for three consecutive years. Researchers estimate that more than half of the world’s reefs experienced moderate or worse bleaching, and about 15% suffered significant mortality. Repeated heatwaves left little time for recovery, compounding long-term damage.
Bleaching is fundamentally a breakdown of symbiosis. Corals rely on their algae for up to 90% of their energy. When heat disrupts photosynthesis, the algae produce harmful molecules, prompting the coral to expel them. Deprived of food, corals enter physiological stress. Some recover if conditions improve; others succumb to starvation, disease, or algal overgrowth. Recovery, if it occurs, can take decades.
Mass bleaching often follows large-scale climate patterns such as El Niño. Not all reefs suffer equally. Local factors — depth, currents, water clarity, and past exposure to temperature swings — influence resilience. Some reefs function as temporary refuges, while others are highly vulnerable.
Future projections suggest worsening conditions. Many reefs may experience longer bleaching seasons and more frequent heat stress. These projections are already materializing: a Fourth Global Coral Bleaching Event began in 2023, affecting multiple ocean basins.
Bleaching is only one pressure among many. Reefs also face:
- Ocean acidification, which weakens coral skeletons
- Overfishing, allowing algae to overrun reefs
- Pollution and sediment runoff
- Physical destruction from mining, dredging, and development
These stresses reduce the ability of reefs to recover after heat events.
Reefs matter beyond biodiversity. They support fisheries, tourism, and coastal protection for hundreds of millions of people. By dissipating wave energy, they reduce erosion and storm damage. If reefs decline, shorelines become more exposed.
Some corals show resilience, particularly in naturally variable environments, and scientists are exploring restoration and experimental interventions. Yet most experts agree these efforts can only buy time locally. The long-term outlook depends largely on global warming. Coral bleaching is not a single catastrophe but a recurring stress reshaping reefs unevenly across the tropics — leaving a narrowing margin for survival in a rapidly changing ocean.
The full story: Coral bleaching: How warming seas are transforming the world’s reefs