Why do coral reefs bleach when the water gets too warm?
Quick Answer
Heat stress makes corals expel the symbiotic algae they rely on for food and color, so the white limestone skeleton shows through. If stressful conditions last too long, the coral can die.
The Full Story
Most reef-building corals live in partnership with microscopic algae called zooxanthellae, which provide much of the coral’s energy through photosynthesis. When seawater stays unusually warm, the relationship breaks down and the coral expels the algae, a process called bleaching. Corals in many tropical regions start bleaching after only about 1–2°C above their usual summer maximum for long enough. The Great Barrier Reef, first described by British explorer James Cook in 1770, has suffered repeated mass bleaching events in recent decades, including major events in 1998, 2016, 2017, and 2024. Bleaching does not always kill coral, but repeated heat stress makes recovery much harder.
Key Facts
YouTube Angle
Suggested video title for this topic:
"The hidden reason coral reefs turn ghostly white"