Stress Onset metrics: Knowing the timing and variability of heat stress historically helps marine resource managers define their 'response window'. This is the period of heightened alertness that precedes a coral bleaching event; i.e., the time when managers and decision makers are preparing their response to and broad communication about an impending event. Heat stress events are defined for 1985-2023 by applying Coral Reef Watch's Degree Heating Week (DHW) methodology at coral reef-containing and adjacent locations worldwide, using the Version 3.1 daily global 5km CoralTemp satellite sea surface temperature (SST) data product. Stress Onset metrics are calculated for the significant (reef-wide) coral bleaching-level threshold for heat stress (i.e., DHW≥4).

Mode Onset [most frequent month of onset, year-round, globally] and Onset Variability [range: 0.0-174.0 days]: The two metrics presented here indicate the most frequently occurring (mode average) month of onset for significant (reef-wide) coral bleaching-level heat stress (i.e., DHW≥4), at each location, during 1985-2023, and its variability, calculated as the standard deviation of the mode onset values. These should be interpreted based on the number of such events in the data record (see Stress Frequency).

Earliest Onset [by month, year-round, globally]: This metric indicates the earliest month in a year, during which the onset of significant (reef-wide) bleaching-level heat stress (i.e., DHW≥4) was experienced, at each location, from 1985-2023.