The maps above show the accumulated coral heat stress in the tropical Atlantic and the Caribbean during a 12-week period ending September 30, 2010 (left) and 2005 (right). Areas shown in dark blue have not accumulated thermal stress over the previous 12 weeks, meaning that the temperature had not crossed the local bleaching threshold. Shades of pink indicate areas where corals experienced a build-up of thermal stress that was enough to cause significant bleaching (light pink) or mass bleaching and death (dark pink). A “degree heating week” is equivalent to 1 week of temperatures that are 1°C higher than the normal summertime maximum. Bleaching becomes likely above 4°C-weeks; widespread mortality is likely after 8.