Map showing the approximate locations of major coral reefs (blue). Also shown are sea surface temperatures from December 2017, with blue being the coldest and pink and yellow the hottest. From 2014 to 2017, rising sea temperatures have caused the worst coral bleaching on record, with over 70 percent of reefs experiencing damage. When corals are stressed by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae living in their tissues, causing them to turn completely white.