Coral Bleaching Risk Calculator

NOAA's real Degree Heating Weeks metric sums the actual satellite-measured weekly anomaly for each of the past 12 weeks, which naturally varies week to week as ocean temperatures fluctuate. This calculator simplifies that into one multiplication of a single entered anomaly by the number of weeks above threshold, assuming a roughly constant anomaly throughout — a reasonable approximation for a steady heat event, but less precise for a period with sharply rising or falling temperatures.

A Watch signals that sea surface temperatures have crossed the bleaching threshold but accumulated heat stress remains low, meaning coral is under some thermal stress but significant bleaching isn't yet expected. A Warning means accumulated stress has grown enough that visible bleaching is becoming increasingly likely if conditions persist, marking a meaningfully higher-risk stage than a Watch alone. These map directly onto NOAA Coral Reef Watch's own published DHW thresholds for its 5km satellite product: a DHW of 4°C-weeks or more is where NOAA flags a bleaching risk (Watch), and 8°C-weeks or more is where it expects reef-wide bleaching with mortality of heat-sensitive corals (Warning).

Not automatically or quickly — the Recovery Time estimate reflects that bleached coral has expelled its symbiotic algae and needs time to recolonize and rebuild energy reserves even after stressful temperatures subside. Severely bleached coral can die outright rather than recover, and repeated bleaching events before full recovery compound the damage to a reef.

This calculator treats the anomaly you enter as roughly constant across the weeks above threshold, which works as an approximation for a sustained marine heatwave but won't capture a period where temperatures fluctuated significantly week to week. For a more variable event, entering the average anomaly across the affected weeks gives a more representative DHW estimate than either the peak or the lowest reading alone.