The NOAA Coral Reef Watch (CRW) daily 365-day Bleaching Stress Extent product summarizes the spatial extent of bleaching-level heat stress accumulations across coral reef locations over a full annual cycle (365 days) that is presented as the daily 5km 365-day Maximum Bleaching Alert Area product. The product is presented in two versions of two-panel time series figures: 2020–present (for most recent years, top figure) and 1986–present (for the entire time series, bottom figure). Each of the two figures shows the percentage of 5 km reef-containing pixels within each of the three tropical ocean basins (Pacific, Atlantic, Indian; top panel) and globally (bottom panel) that have experienced bleaching-level heat stress (Bleaching Alert Level 1 or higher) at any time during a rolling 365-day period.

Data and figures are updated daily, with each dated daily value representing the cumulative percentage of reef pixels exposed to bleaching-level heat stress during the 365 consecutive days up to and including the date. Thresholds of 20% (red dashed line) and 12% (black dashed line) are shown for their use in identifying periods of active global bleaching stress (see below).

At Alert Level 1 of Coral Reef Watch's bleaching heat stress alert system, accumulated stress has reached levels associated with a risk of widespread coral bleaching. This product provides a measure of the geographic footprint of ecologically relevant heat stress. By integrating conditions over a full annual cycle, the Bleaching Stress Extent metric captures a snapshot of the global state of recent significant thermal stress. SST variability at 5 km spatial and daily temporal resolution can result in substantial day-to-day variation in heat stress at individual reef sites. While CRW's operational daily Bleaching Alert Area and Degree Heating Week products provides crucial information on current near-term conditions, the daily 365-day Bleaching Stress Extent product provides critical information on cumulative basin- and global-scale progression of bleaching-level heat stress over a full annual cycle (i.e., 365 days). In effect, this provides a cumulative, time-integrated perspective on global bleaching heat stress exposure focusing on annual cycle instead of seasonal cycle which is not synchronized among global reefs.

This product is consistent with the framework described in Spady et al. (2026, Coral Reefs), where the proportion of reefs experiencing bleaching-level heat stress within a one-year window is used to help characterize the scale and severity of thermal stress during global coral bleaching events. Historically, global-scale coral bleaching events have coincided with periods during which 20% or greater of the world’s reef pixels have experienced Alert Level 1 conditions or worse within a one-year period. Criteria for periods of active global bleaching stress are set forth in Spady et al. (2026) as:

Note that the 365-day Bleaching Stress Extent presented here is renamed Annual Bleaching Stress Extenet introduced in the paper, for a better indication of the product content.