Corals are extremely sensitive to water that is too warm-even temperatures just 1°C above the highest average summertime temperature. If corals bathe in water above this critical threshold for just four weeks (or at higher temperatures for even shorter durations), the accumulated heat stress can induce coral bleaching, a condition where coral polyps expel their beneficial algae and starve. Bleached coral turns white and can die or remain weakened for years. Although coral can bleach for reasons other than warm water, in recent decades a worrisome pattern has emerged. Episodes of global coral bleaching are becoming more frequent. These widespread events are thought to be among the earliest distinct signs of climate change's effects on Earth's organisms.
According to NOAA scientists, 2010 tied with 2005 as the warmest year on record. Throughout the year, satellite monitoring from NOAA's Coral Reef Watch program detected that sea-surface temperatures exceeded the bleaching threshold for several weeks in various regions of the world. Scientists and reef managers soon began to observe excessive bleaching in many areas in which it was predicted by satellite. The 2010 global bleaching event was the second ever recorded.