Why Reefs Are Turning White — and How We Can Help
Coral reefs are biodiversity powerhouses that protect coasts, support fisheries and tourism, and feed hundreds of millions of people. Coral bleaching — when corals lose their symbiotic algae and turn white — is the most urgent threat to their survival. Bleached corals aren't dead, but they’re weakened and more likely to die if stress persists.
What Is Coral Bleaching?
Corals host microscopic algae (zooxanthellae) that provide most of their energy and vibrant color. When water gets too warm or other stressors hit, corals expel these algae. Without them, corals appear white and can starve unless conditions improve and the algae return.
Main Causes
- Marine heatwaves: Just 1–2°C above the usual summer maximum for several weeks can trigger mass bleaching. Strong El Niño years make this worse.
- Ocean acidification: CO₂ absorbed by the ocean lowers pH, making it harder for corals to build skeletons and recover.
- Pollution & sediment: Nutrients, sewage, and runoff fuel algal blooms and reduce light; sediments smother corals.
- Overfishing & damage: Removing key species and physical damage from anchors or destructive fishing increase stress.
Why It Matters
- Biodiversity loss: Reefs support ~25% of marine species despite covering <1% of the seafloor.
- Economic risk: Fisheries, tourism, and coastal protection from reefs are worth many billions annually.
- Food security: Reef fisheries are crucial protein sources for many coastal communities.
- Coastal safety: Healthy reefs absorb wave energy and reduce storm damage.
Recent Global Events
The Great Barrier Reef, the Caribbean, the Red Sea, and the Indian Ocean have suffered multiple mass bleaching events since 1998, with unprecedented marine heatwaves in recent years affecting reefs worldwide.
What Works — From Local to Global
- Cut greenhouse gases by accelerating clean energy and efficiency — the only lasting solution to heat-driven bleaching.
- Protect water quality via better wastewater treatment, stormwater management, and agricultural best practices.
- Strong fisheries management and no-damage zones to keep reef food webs intact.
- Marine protected areas to reduce local stress and give reefs recovery space.
- Restoration & assisted evolution: Grow and outplant resilient corals, and pilot heat-tolerant strains where appropriate.
Bottom Line
Coral bleaching is a climate story — but it's also a local management story. Every degree of warming avoided and every local stress reduced increases the odds that reefs survive and recover.