When ReefBeacon says a reef is under heat-stress alert, the decision rests on one number: Degree Heating Weeks, or DHW. It is worth understanding, because it is the difference between “the water is warm” and “the corals are in trouble”.
From temperature to accumulated stress
Sea surface temperature (SST) alone says little about stress — a reef that lives at 30 °C is comfortable at 30 °C. What matters is how far and how long the water sits above the reef’s usual summer maximum. NOAA Coral Reef Watch measures that excess as HotSpot, the amount by which today’s temperature exceeds the local bleaching threshold, and accumulates it over time. One degree above the threshold, sustained for a week, is one Degree Heating Week. Half a degree for two weeks is also one. The number adds up as a heatwave persists — and resets only slowly as the ocean cools.
Why 4 and why 8
The thresholds ReefBeacon applies are the international scientific convention, not a house invention: DHW above 4 corresponds to significant accumulated stress, where bleaching becomes likely — that raises a heat-stress alert. Above 8, the stress is severe enough that widespread bleaching and mortality become likely — that marks high bleaching risk. Because the scale accumulates, a long mild heatwave can end up more dangerous than a short hot spike.
What DHW does not tell you
DHW describes the heat, not the corals. Two reefs with the same reading can bleach differently depending on species, history, currents or cloud cover. That is why ReefBeacon alerts are labeled as heat-stress warnings, not real-time confirmation of bleaching — satellites see the ocean’s fever, and only a field survey can see the patient.
How ReefBeacon uses it
Every day the platform reads NOAA’s 5-km DHW product for each of its 65 stations, checks the values against what NOAA publishes, and applies the two thresholds with deterministic code — the same rule, every reef, every day, with no model discretion and no manual override. When a threshold is crossed, the alert that reaches coordinators within 30 minutes cites the readings behind it, so anyone can trace the decision back to the source.