What was done
This paper reviews the subject of mass coral reef bleaching, attempting to explain why this phenomenon has been so dramatic over the past couple of decades.

What was learned
The authors note that empirical evidence indicates that major bleaching episodes appear to be synchronized with El Niņo events that occur every three to four years; and they suggest that when these high-temperature events are superimposed upon a rising baseline temperature due to global warming, they induce coral bleaching.

What it means
The authors acknowledge some important facts about this matter that must not be forgotten.  First, they state that "it is possible that mass coral reef bleaching has been occurring unobserved throughout the last centuries but has only been noticed in recent decades."  Also, they note that although the recent global warming trend is "the most widely accepted explanation" for the apparent increase in coral bleaching over the last few decades, "it should be viewed more as an untested hypothesis, supported so far only by circumstantial evidence."  Hence, there is much that remains to be resolved about the matter of the apparent recent rise in mass coral reef bleaching events.