Stress Onset metrics: Knowing the timing and variability of heat stress in the past 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 1985-2021 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 bleaching-level threshold for heat stress (DHW≥4).

Where relevant, the range of values calculated from analyzed pixels is shown in [brackets] after each metric name.

Mean Onset and Onset Variability [0.0-250.0 days]: We determined the start date of each heat stress accumulation (during 1985-2021) that led to significant bleaching-level exposure (DHW≥4). These metrics indicate the average timing of stress onset (by month), and how much that has varied, calculated as the standard deviation of mean onset values. These should be interpreted based on the number of such events in the data record (see Stress Frequency). For Onset Variability (or variability in the mean onset), the color scale is divided into quantiles, such that each color represents an approximately equal number of locations.

Earliest Onset: Using the same compilation of bleaching-level exposure events, this metric indicates the earliest month (between 1985-2021) during which the onset of significant bleaching-level heat stress was experienced.