Coral bleaching is a sign of stress in corals that reveals itself as a loss of color. Corals that form a reef have a symbiotic relationship with an algae called zooxanthellae, which lives in the coral tissue; these algae give a coral its color, and under stress, corals may release zooxanthellae, thus losing its color.
Coral stress may be a result of changing water temperature, increased solar light levels, a change in water chemistry, or a change in nutrients, among other factors.
Once a bleaching event takes place, corals will continue to bleach even without the stressor. The coral colony may survive, but time is required for it to "heal." If a colony does heal, it may be inhabited by a different species of zooxanthellae; some corals host different types of zooxanthellae.
The ability to withstand stress and recovery time from a bleaching event varies among coral species. Large corals are able to weather extreme temperature change, while more fragile corals are more susceptible to bleaching and thermal stress. Local climatic conditions also influence the risk of bleaching, as does the health of the coral and the zooxanthellae.
The primary cause of bleaching is high water temperature. If water temperature is increased by 1.5 degrees to 2 degrees Celsius for six to eight weeks, bleaching occurs; if the temperature remains high beyond eight weeks, the coral will start dying.
Another cause of bleaching is increased solar radiation. Photosynthetically active radiation and Ultra-violet (UV) radiation are both linked to coral bleaching.
A change in water chemistry as simple as dilution from precipitation can cause coral bleaching events in small shore areas.
The exposure of a reef coral colony to the atmosphere due to, for example, extremely low tides can induce bleaching. The stress of the exposure, on top of any other stress, may cause the loss of zooxanthellae, or lead to coral death.
Global warming is a driving force behind coral bleaching. It is responsible in part for the change in sea water temperature, and the depletion of the ozone layer increases solar radiation, especially of UV rays. As reef corals approach the upper limits of their tolerance levels, small temperature changes over weeks, or larger increase over days will lead to coral death. As the temperature rises, so rises the sea level; a rising sea level would cause coral to try to grow faster, or kill coral by drowning or lower light levels.
It has been estimated that global warming could increase coral bleaching even more, driving coral mortality over 95% in some regions, including extinctions.
Source: Jason Buchheim, "Coral Reef Bleaching," marinebiology.org.