What was done
The author of this paper reviews existing knowledge of potential causes of coral reef bleaching.

What was learned
In the words of the author, coral reef bleaching "is a general response to stress, [and] it can be induced by a variety of factors, singly or in combination."  The leading factors responsible for large-scale bleaching, according to the author, are "elevated sea temperatures and high solar irradiance (especially ultraviolet wavelengths), which may frequently act jointly."  However, the author notes that anomalously low water temperatures can induce coral bleaching as well as high temperatures can, indicating that "bleaching events occur during sudden temperature drops that accompany intense upwelling episodes, thermocline shoaling or seasonal cold-air outbreaks."

As for the fate of coral reef ecosystems if earth's temperature were to continue to rise, the author points out that "numerous reef-building coral species have endured three periods of global warming, from the Pliocene optimum (4.3-3.3 million years ago) through the Eemian interglacial (125 thousand years ago) and the mid-Holocene (6000-5000 years ago), when atmospheric CO2 concentrations and sea temperatures often exceeded those of today."  He also notes that others have concluded that "an increase in sea warming of less than 2°C would result in a greatly increased diversity of corals in certain high latitude locations."

What it means
This study demonstrates that there are a number of factors involved in determining the magnitude and extent, as well as the causes, of coral bleaching events, although water temperature and solar irradiance appear to have a more significant impact on large-scale bleaching events.  However, the author states that "despite recent incidences of severe coral reef bleaching and mortality, no species' extinctions have yet been documented."  However, with or without significant warming or increases in irradiance levels, the author notes that "coral reef degradation from anthropogenic pollution and overexploitation will still continue."