What is the main cause of mass coral bleaching on the Great Barrier Reef?

Abnormally warm seawater is the main cause of mass coral bleaching on the Great Barrier Reef.

Reef-building corals live in partnership with microscopic algae called zooxanthellae. These algae provide much of the coral’s energy through photosynthesis and give many corals their colours. When water stays unusually warm, corals become stressed and expel the algae, leaving their tissues visibly pale or white.

Bleaching does not always mean that a coral has died. If temperatures fall and conditions improve, some corals can regain their algal partners. However, prolonged or repeated heat stress can cause extensive mortality, especially when combined with disease, poor water quality or limited food. Other stresses can bleach corals too, but heat is the principal driver of large regional events.