Corals experience thermal stress, the main cause of bleaching, when sea surface temperatures exceed 1°C (1.8°F) above the maximum summertime mean. This stress worsens as the heat anomaly persists. Degree Heating Week (DHW) shows how much heat stress has accumulated in an area over the past 12 weeks (3 months) by adding up any temperature exceeding the bleaching threshold during that time period. When DHW reaches 4°C-weeks (7.2°F-weeks), significant coral bleaching is likely, especially in more sensitive species. When DHW is 8°C-weeks (14.4°F-weeks) or higher, widespread bleaching and mortality from thermal stress may occur.