Background
The authors report "it has been suggested that since massive corals, some as old as 700 years, died as a result of the 1998 bleaching event, it must have been the most severe bleaching event to hit the Great Barrier Reef over the last seven centuries (Hoegh-Guldberg, 1999)."

What was done (1)
Questioning the validity of the simplistic basis of Hoegh-Guldberg's contention, Hendy et al. considered "the likelihood of observing, in cores taken from Porites colonies, past mass coral mortality events equivalent in intensity and scale to the 1998 bleaching event," noting that "an historic record of past coral mortality events is needed to gain some perspective on current events and the impact of recent environmental change."  This exercise included the careful examination of eight long Porites cores extracted from inshore and midshelf reefs in the central Great Barrier Reef.

What was learned (1)
The researchers discovered two hiatuses in coral skeletal growth that were accurately dated to 1782-85 and 1817 A.D.  The telltale "die-off scars" were observed in only one core for each event.  Contemporary historical and proxy-climate records indicate that El Niņo conditions occurred at the times of both growth discontinuities, with those of 1782-83 being termed "exceptional" by Whetton and Rutherfurd (1994).  Other data indicated that low salinity from river runoff was a contributor to bleaching during the 1817 event.  The authors note that similar environmental conditions were associated with the 1998 bleaching of the Great Barrier Reef.

What was done (2)
Based on the work of Marshall and Baird (2000), who studied the bleaching responses of different coral taxa to the environmental conditions that produced the 1998 event in the Great Barrier Reef, Hendy et al. calculated the probability of sampling an event of equivalent severity that may have occurred in the more distant past.

What was learned (2)
The authors' analysis indicated that "the chance of seeing an event across all [eight] cores is exceedingly unlikely, even for one as dramatic as the 1998 bleaching event."  In fact, they calculate that "a growth discontinuity is most likely to be observed in only one of the cores in any sample population size smaller than 17 cores."

What it means
In the first instance, Hendy et al. conclude that Porites colonies can recover and continue growing for centuries after a partial mortality event such as that experienced on the Great Barrier Reef in 1998, while their work suggests that similar bleaching events may have occurred in the late 18th and early 19th centuries.  In the second instance, their study again makes it clear that such coral bleaching events may well have occurred periodically in past centuries, but that they may be difficult to detect without massive multiple-coring studies.  Until such work is conducted, therefore, we will not be able to accurately assess the uniqueness of the 1998 bleaching event, although there is now some reason to believe it may not have been as unusual as climate alarmists claim.

References
Hoegh-Guldberg, O.  1999.  Climate change, coral bleaching and the future of the world's coral reefs.  Marine and Freshwater Research 50: 839-866.

Marshall, P.A. and Baird, A.H.  2000.  Bleaching of corals in the Great Barrier Reef: differential susceptibilities among taxa.  Coral Reefs 19: 155-163.

Whetton, P. and Rutherfurd, I.  1994.  Historical ENSO teleconnections in the eastern hemisphere.  Climatic Change 28: 221-253.