Coral Bleaching Risk Calculator

Degree Heating Weeks measure accumulated thermal stress in units of degrees C-weeks. The reference point is the maximum monthly mean, the warmest month of the local long-term sea surface temperature climatology, so a reef adapted to 29 degrees C is judged against its own baseline rather than an absolute threshold. The instantaneous anomaly above that baseline is the HotSpot; DHW is the sum of HotSpots over a rolling twelve-week window. NOAA Coral Reef Watch counts only anomalies of 1 degree C or more toward the accumulation, on the observation that smaller excursions fall within normal tolerance. The operational thresholds are well established: 4 DHW marks Alert Level 1 with significant bleaching expected, and 8 DHW marks Alert Level 2 with widespread bleaching and mortality likely. The pH term reflects experimental evidence that corals under acidified conditions spend more energy on calcification and bleach at slightly lower temperatures; surface ocean pH has fallen from roughly 8.17 before industrialisation to about 8.05 today. This implementation simplifies in ways worth knowing. It multiplies a single constant HotSpot by a duration instead of summing week-by-week anomalies from a real temperature series, and it does not apply the 1 degree C accumulation floor, so a persistent 0.5 degree anomaly registers stress here where the operational product would record none. The acidification and light contributions to the composite index are weighted heuristically rather than derived from a calibrated dose-response model, and the recovery estimate in months is indicative only. Real outcomes also depend on species composition, symbiont type, water clarity, prior bleaching history and local water quality, none of which appear as inputs.

Corals were known to host symbiotic dinoflagellates well before anyone worried about losing them, but bleaching became a research field after the strong 1982 to 1983 El Nino, when Peter Glynn documented catastrophic coral mortality in Panama and the Galapagos and linked it directly to sustained warm water rather than disease or pollution. Through the 1980s the availability of satellite sea surface temperature from the AVHRR sensors made large-scale monitoring feasible, and researchers at NOAA, notably Alan Strong and colleagues, developed the HotSpot and Degree Heating Week products during the 1990s to convert those temperature fields into a stress metric reefs managers could act on. The 1997 to 1998 El Nino produced the first recognised global bleaching event, affecting reefs across every tropical ocean basin and killing an estimated sixteen percent of the world's coral. NOAA Coral Reef Watch was formalised around 2000 and now issues near real-time global alerts. Ove Hoegh-Guldberg argued in 1999 that bleaching would become an annual event on most reefs within decades if warming continued, a projection that has broadly tracked observations. A second global event accompanied the 2010 El Nino, and the third, running from 2014 into 2017, was the longest and most damaging on record, with back-to-back bleaching on the Great Barrier Reef in 2016 and 2017. Taxonomic understanding advanced too: the symbionts long lumped under the genus Symbiodinium were split into the family Symbiodiniaceae by LaJeunesse and colleagues in 2018, clarifying why heat tolerance differs so sharply between coral colonies.