Coral bleaching occurs when sea temperatures exceed the local maximum by just 1°C for 4+ weeks. Heat stress causes symbiotic algae (zooxanthellae) to produce toxic reactive oxygen species — the coral expels them, losing 90% of its energy supply and revealing its white calcium carbonate skeleton.
NOAA's DHW metric accumulates temperature anomalies: DHW = Σ(SST − bleaching threshold) over 12 weeks. DHW > 4°C-weeks triggers bleaching; DHW > 8°C-weeks causes significant mortality. The Great Barrier Reef has experienced six mass bleaching events since 1998.
If temperature stress ends quickly, zooxanthellae can recolonize within weeks. Full recovery takes 10–15 years. Repeated bleaching prevents recovery — the GBR's 2016 and 2017 back-to-back events killed 50% of shallow corals. Thermally tolerant symbiont strains (clade D) offer partial resilience.