James Cook University - Reef Dieoff Worst In At Least 11,000 Years - Mongabay

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Two new studies by scientists at the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University suggest that coral reefs may be in worse shape than previously thought. The first, appearing in the journal Geology indicates that the current large scale coral die-offs are now occurring more frequently than at any time in the last 11,000 years. The second, published in Current Biology, suggests that the loss of a single "keystone" species can trigger a rapid shift in the health of a reef.

The first study, led by Associate Professor John Pandolfi of the Australian Research Council Centre of Excellence in Coral Reef Studies and The University of Queensland, examined fossilized reefs of the Huon Peninsula in Papua New Guinea and determined that past reef die-offs occurred about every 1500 years due to some catastrophic event -- a rate that is exceeded by the current decline in coral reefs;

“The cause of some of these events was volcanic, but others may have been due to bleaching, disease, or something else - we just don't know. Regardless, what is clear is that the frequency of die-off was so much lower than it is today. The frequency of reef events in the fossils is at least an order of magnitude less than it is today” said Pandolfi.

Pandolfi said the results show that the ancient reefs "recovered rapidly after these events, taking as little as 100 years to be repopulated by the corals that normally occurred there. The recovery of the Great Barrier Reef from the devastating impact of the crown of thorns starfish took less than a few decades, at least in part due to comprehensive reef management,” Pandolfi continued,”…but this rate of recovery isn't seen in other parts of the world….some reefs still haven't recovered from .”

EDIT

http://news.mongabay.com/2006/1218-reefs.html