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Abstract Sea-surface temperature (SST) extremes are a major stress to coral reef ecosystems, yet most global SST products lack the spatial resolution to measure fine-scale nearshore gradients that influence reef exposure. Using the 5 km NOAA geo-polar blended SST record and a 30 year climatological baseline (1985–2014), we updated the local extreme heat index (LEHI), a metric based on pixel- and month-specific thresholds, to quantify changes in the frequency of extreme thermal events across 533 coastal grid cells in the U.S. Pacific Islands. The mean LEHI increased 14-fold from 1985–1994 to 2015–2024, and 6.4-fold relative to 2005–2014, indicating that extreme thermal conditions once considered rare are now common. The average LEHI increased by 0.07 units per decade (range: 0.01–0.11), equivalent to one additional month of heat stress every ∼15 years under a linear trend. Regional means for 2015–2024 ranged from 0.18 ± 0.03 (Wake) to 0.42 ± 0.08 (Marianas), with Guam peaking at 0.67 in 2020 (8/12 extreme months). Annual spatial LEHI correlated strongly with NOAA Coral Reef Watch metrics (zero-lag r = 0.70 for the seven-day maximum bleaching alert area; r = 0.65 for degree-heating weeks). Cross-correlation peaks at lags −1 to −4 years indicate that elevated LEHI generally precedes accumulated heat stress, representing an earlier statistical signal. High-resolution LEHI complements existing operational products by highlighting areas of persistent exposure (e.g., Palmyra), identifying potential thermal refugia (e.g., Wake Island), and extending lead times for management actions.
Tanaka et al. (Fri,) studied this question.