The thermal gap constraint: why satellite degree heating weeks fail where reefs need them most

Degree heating weeks (DHW), the global standard for coral bleaching prediction, shows large regional variation in skill. Whether this reflects data quality or an architectural limitation is unresolved. Using Japan’s Monitoring Site 1000 (26 sites, 5 years) and the Global Coral Bleaching Database (7,286 records, 51 countries), I identify a thermal gap constraint: as climatological SST rises above ~28.5°C, anomaly accumulation compresses, preventing DHW from reaching alert thresholds. Metric architecture dominated prediction skill over SST product choice (Δ AUC = 0.24 vs 0.02). DHW’s AUC declined from 0.705 to 0.568 globally. An absolute-temperature metric bypassed the constraint (AUC = 0.884 vs 0.624). With a reef mask applied, 63% of global reef area falls within the constraint zone (29% across all warm-ocean pixels). A two-tier prediction approach is warranted.