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 the impending event. Heat stress events are defined for 1985-2018 by applying Coral Reef Watch's Degree Heating Week (DHW) methodology at coral reef-containing and adjacent locations worldwide using the Version 3.1 'CoralTemp' daily global 5km satellite sea surface temperature (SST) data product. Stress Onset metrics are calculated for the 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 [0.6-254.6 days]: The start date of each heat stress accumulation (1985-2018) that led to bleaching-level exposure (DHW≥4) was determined. These metrics indicate the average timing of stress onset (by month) and how much that has varied, calculated as the standard deviation of onset values. These should be interpreted based on the number of such events in the data record (see Stress Frequency). For Onset Variability the color scale is divided into quantiles (i.e., 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 (1985-2018) during which the onset of bleaching-level heat stress was experienced.