What is coral bleaching?

Corals have a symbiotic relationship with microscopic algae called zooxanthellae, which provide up to 90 percent of the coral’s energy through photosynthesis and give corals their vibrant colours. When water temperatures rise by just 1 to 2 °C above normal summer maximums, the algae produce toxic levels of oxygen radicals, and the coral expels them to protect itself. Without the algae, the coral’s white limestone skeleton shows through. If the stress is short, the algae can return and the coral survives; if it persists, the coral starves and dies. Mass bleaching events have become more frequent: the Great Barrier Reef experienced back-to-back events in 2016, 2017, 2020, 2022 and 2024.