Read here.
"Characteristic bleaching scars and isotope temperature records from coral cores commonly show evidence of past bleaching events going back thousands of years. There is no evidence for a recent increase in frequency and/or severity of bleaching events and nothing to link extended periods of calm winds with global warming...In past geologic periods when global climate was warmer than at present corals enjoyed greater latitudinal distribution. The most likely effect of a warming climate on reefs would seem to be an expansion of their geographic distribution and there is some evidence this is already happening."
