What was done
This paper reviews the scientific literature dealing with coral bleaching events.

What was learned
The author notes that the key environmental variables that cause coral bleaching remain "poorly defined," mostly due to conflicting environmental triggers thought to elicit bleaching events.  Environmental variables that have been identified with coral bleaching in the scientific literature include (1) elevated seawater temperature, (2) decreased seawater temperature, (3) solar radiation, (4) a combination of solar radiation and elevated temperature, (5) reduced salinity, and (6) bacterial infections.

Concerning elevated seawater temperatures, which are often claimed to be the primary cause of coral bleaching, the author notes that "attributing cause and effect has not been straightforward, with an apparent lack of correlation between coral reef bleaching and sea surface temperature anomalies in the tropical Western Atlantic."  In addition, Brown points out that the common usage of satellite-derived sea surface temperatures in this regard is problematic, noting a lack of resolution such that "better than 0.5°C cannot be achieved" using the satellite data.  He further notes that satellite data "only give [sea surface temperature] values relating to the top few millimeters of surface waters which may relate more closely to changes in solar radiation than bulk sea temperature."  Another problem with the elevated temperature hypothesis is "the frequent pattern of coral colonies bleaching only in their upper surfaces," which would seem to suggest some sort of solar radiation effect.

What it means
This review of the scientific literature weakens the oft-cited claim that global warming is responsible for recent coral bleaching events, while it strengthens the likelihood of a solar radiation connection.