"During the most recent reefwide bleaching events, in 1998 and 2002,…

"During the most recent reefwide bleaching events, in 1998 and 2002, bleaching affected at least half of the system’s reefs (50 percent and 60 percent respectively). Yet in each case, only 5 percent of reefs suffered permanent damage. Dead coral at 150 reefs is not insubstantial, but in the end, nearly all of the bleached coral recovered."

http://www.slate.com/articles/health_and_science/science/2016/05/here_s_how_the_great_barrier_reef_could_still_recover.html