Coral Bleaching Threshold Calculator

The bleaching threshold is defined as the Maximum Monthly Mean (MMM) sea surface temperature for a given reef location, plus 1°C. When ocean temperatures exceed this threshold for a sustained period, corals expel their symbiotic algae (zooxanthellae), causing bleaching. NOAA Coral Reef Watch uses this standard to issue global bleaching alerts.

A HotSpot is the difference between the current sea surface temperature (SST) and the bleaching threshold (MMM + 1°C). If the SST is above the bleaching threshold, the HotSpot value is positive, indicating thermal stress. Only positive HotSpot values contribute to Degree Heating Weeks. A HotSpot of 0 or below means no additional heat stress above the bleaching threshold.

Degree Heating Weeks (DHW) measure the accumulation of thermal stress over time. They are calculated by summing the HotSpot values (temperatures above the bleaching threshold) over a 12-week rolling window, expressed in °C-weeks. A DHW of 4 typically causes bleaching in 30–40% of corals, while a DHW of 8 or above often triggers mass bleaching and significant coral mortality.

NOAA Coral Reef Watch issues four alert levels: Watch (DHW ≥ 0, elevated risk), Warning (DHW ≥ 4, bleaching likely), Alert Level 1 (DHW ≥ 4 + HotSpot ≥ 1°C, severe bleaching likely), and Alert Level 2 (DHW ≥ 8, mass bleaching and significant mortality expected). Alert Level 0 means no significant thermal stress is present.

Shallow reefs (0–5 m) are most vulnerable because they experience greater temperature fluctuations and higher solar irradiance, compounding thermal stress. Deeper reefs (15–30 m) may be partially buffered from surface temperature extremes, though sustained marine heatwaves can still cause bleaching at depth. Depth alone cannot fully protect corals from prolonged high DHW events.

High turbidity can slightly reduce bleaching risk by limiting the amount of solar irradiance reaching corals, which is a co-stressor alongside elevated temperature. However, turbid water also reduces photosynthesis and can introduce other stressors. In practice, turbidity provides only marginal protection and does not eliminate bleaching risk when SST remains well above the bleaching threshold.

NOAA Coral Reef Watch uses daily 5 km resolution satellite SST data to monitor thermal stress on coral reefs globally. The satellite-derived SST is compared to the climatological MMM for each reef pixel, generating HotSpot and DHW products. These are updated daily and used to issue bleaching alerts weeks before bleaching events are confirmed in the field, enabling early management responses.

Yes, bleached corals can recover if thermal stress subsides quickly enough. Recovery typically takes 8–12 weeks after temperatures return to normal levels. However, corals that remain bleached for more than 4–6 weeks face significantly higher mortality. Repeated bleaching events in successive years can prevent full recovery and lead to long-term reef degradation, as seen on the Great Barrier Reef.