The reef has suffered eight mass coral bleaching events since 1979, triggered by unusually high water temperatures. However, annual bleaching is expected by mid-century if our heat-trapping emissions continue at their current pace, thus leaving the reef vulnerable to diseases from which it may not recover Ocean acidification expected to occur if atmospheric carbon dioxide surpasses 500 parts per million is likely to limit the capacity of the reef to recover from bleaching events, and to cope with other stresses.