More severe bleaching events have dramatic long-term effects on the coral. The ability of the coral to feed itself in the absence of zooxanthallae may be very important to its survival during and after a bleaching event. Recovery rates appear to differ, however, with species, and the time required to attain full recovery of symbiotic algae may vary from as little as 2 months to as much as one year. When the level of environmental stress is high and sustained the coral may die.

The bleaching events reported prior to the 1980s were generally attributed to localized phenomena such as major storm events, severe tidal exposures, sedimentation, rapid salinity changes, pollution, or thermal shock. The events since 1980 have not been so easily explained. Numerous laboratory studies have shown a direct relationship between bleaching and water temperature stress. Elevated water temperatures have been implicated in the majority of the major bleaching events of the 1980s and 1990s.

Coral bleaching related to thermal stress has become much more severe in the last decade. High temperatures associated with the 1997-98 El Niño caused bleaching in much of the world's oceans, with the greatest bleaching in the Indian and western Pacific Oceans. Heating caused major bleaching around the Great Barrier Reef and Northwestern Hawaiian Islands in 2002, and in 2005 we saw the highest heat stress in the Caribbean region of 20 years of satellite monitoring. The 1998 and 2005 events have raised public attention to the plight of coral reefs (see Mark Trail comic at right).