http://www.pnas.org/content/105/45/17442.short

See this paper, hope this helps.

Eric

Possibly, global warming is additional reason, because the temperature for coral living is limited.

Corals are very sensitive organisms, the bleaching problems may be due to combine effects of many adverse environmental factors like Temperature combined with other pollutants.

Bleaching of corals is a sign of stress. As a result the corals loose their zooxanthellae. The corals will stress for a variety of reasons including but not limited to temperature (usually over 30C corals will bleach), salinity (too much fresh water), acidity...