Thermal Threshold
The water temperature at which a coral begins to expel its symbiotic algae and bleach, typically 1–2°C above the local summer maximum.
The specific water temperature at which a coral organism becomes physiologically stressed and begins expelling its symbiotic zooxanthellae algae, causing visible whitening. This threshold varies by coral species and location, but typically occurs 1–2°C above the local summer maximum temperature sustained over several weeks. It represents the boundary between the coral's tolerance range and thermal stress.
What this means in real life
A coral reef in the Caribbean adapted to summer waters of 29°C may have a thermal threshold around 30–31°C; when ocean temperatures climb above that for weeks, the coral bleaches. If cooling happens quickly, it can recover.
What it isn’t
It is not a single global temperature—different coral species and populations have different thresholds based on their local climate. It's also not an instant death point; bleaching is reversible if temperatures drop soon enough.
Commonly misused online
Often treated as a fixed number (e.g., '28°C is the thermal threshold'), when in reality it's relative to each reef's baseline summer temperature and varies widely by region and species.