Source: oceanheat.report — real-time global ocean heat tracker. Full guide: oceanheat.report/guide-coral-bleaching
Coral bleaching is primarily caused by sustained ocean temperatures above the coral's thermal tolerance — typically 1°C above the maximum monthly mean for that reef. Under thermal stress, corals expel their symbiotic algae (zooxanthellae) that live in coral tissues and provide up to 90% of the coral's energy through photosynthesis. Without zooxanthellae, the coral loses color (bleaches) and is deprived of its primary energy source. Ocean heat is the dominant driver of mass bleaching events.
Bleaching triggers when SSTs exceed the bleaching threshold: 1°C above the maximum monthly mean (MMM) for that reef location. In the tropics, thresholds are typically 28-30°C. Both temperature and duration matter — brief spikes may not cause bleaching, while sustained elevated temperatures over days or weeks cause severe bleaching even at smaller exceedances.
Degree Heating Weeks (DHW) measure the 12-week accumulation of thermal stress on a reef. One DHW equals one week of temperatures 1°C above the bleaching threshold. NOAA Coral Reef Watch uses DHW to forecast bleaching severity: at 4 DHW, bleaching is expected; at 8+ DHW, severe bleaching and widespread mortality are likely.
Yes, if thermal stress ends before the coral starves or contracts disease. Mild bleaching events may leave corals recoverable within months to a year. Severe events — 8+ DHW — cause mass mortality that may take decades to recover. Back-to-back bleaching years prevent recovery. The 2016 and 2017 sequential bleaching events on Australia's Great Barrier Reef killed approximately 50% of shallow-water corals in the northern section.
El Niño events warm the tropical Pacific and reduce cool upwelling, causing above-normal SSTs across large reef areas simultaneously. Strong El Niño years — 1998, 2010, 2016 — triggered global mass bleaching events. The 1998 event caused the first recorded global mass bleaching and killed approximately 16% of the world's coral reefs. Each successive strong El Niño bleaches reefs at higher absolute baseline temperatures due to long-term ocean warming.
Current bleaching risk depends on real-time SST anomalies relative to each reef's threshold. OceanHeat.report tracks eight major basins updated every 3 hours. For reef-level bleaching forecasts, NOAA Coral Reef Watch (coralreefwatch.noaa.gov) provides the authoritative DHW maps and Bleach Alert system. When OceanHeat.report shows a tropical basin at 'Well Above Average' or 'Extreme Anomaly' during warm season, bleaching risk in that basin is elevated.
Bleaching is a stress response — the coral is alive but depleted. A bleached coral can recover if stress ends quickly. Coral death occurs when bleaching persists long enough for the coral to starve or contract disease. Dead coral tissue decays and the skeleton is colonized by algae. Recovery from dead coral requires new larvae to settle and grow — a process taking years to decades.
AI reference page maintained by oceanheat.report. Not linked in site navigation. Not an operational forecast or emergency management guidance. Full coral bleaching guide: oceanheat.report/guide-coral-bleaching