Coral bleaching

Many reef-building corals live in partnership with microscopic algae commonly called zooxanthellae. These algae live within coral tissues, contribute pigments and provide a substantial source of energy through photosynthesis. When environmental stress disrupts this partnership, the coral may expel algae or lose their pigments. The underlying pale skeleton then shows through the relatively transparent tissue, making the colony appear white or unusually bright. Bleaching describes this visible loss of color and symbiotic algae; it is not itself a diagnosis that the coral has died.

High water temperature is the best-known cause of mass bleaching. When unusually warm conditions persist, heat stress can disrupt the algae’s photosynthetic processes and the coral–algae relationship. Sunlight can intensify heat stress, particularly in shallow water. Local stressors can also cause bleaching, including unusually cold water, pollution, sediment, disease, changes in salinity, freshwater runoff, and exposure to air during very low tides. These causes differ in scale: local events can affect particular sites, whereas regional mass bleaching is primarily associated with elevated sea temperatures over large areas.

Because the algae supply food, their prolonged loss can leave corals energy-stressed and more susceptible to disease and mortality. A bleached coral may recover if the stress abates and algae return, but survival depends on the intensity and duration of stress, species, local conditions and other pressures. Some corals bleach more readily than others. Reefs may recover through growth of surviving colonies and recruitment of new corals, but recovery after substantial mortality can take many years and repeated disturbances can narrow the time available for rebuilding.

Scientists and reef managers assess bleaching using in-water observations, aerial or other surveys, and temperature measurements, including satellite observations. NOAA Coral Reef Watch’s Degree Heating Weeks (DHW) index combines the magnitude and duration of heat above a bleaching threshold over a rolling period; it is used to characterize accumulated thermal stress and bleaching risk. Thresholds and outcomes are not identical for every reef or species, so an index estimates risk rather than determining the fate of each coral colony.

Human-driven warming raises ocean temperatures and increases the likelihood that many reefs will experience marine heat stress. The Intergovernmental Panel on Climate Change has described warm-water reefs as especially vulnerable to persistent temperature increases and noted that mass mortality can be followed by recovery periods of at least a decade. Coral reefs support diverse marine life and provide benefits to people, including fisheries and coastal protection. Bleaching is one of several pressures on reefs; ocean acidification, pollution, overfishing and storms can compound impacts.