A coral and its algae can protect themselves to a point during normal summers. First they dove off Key Largo National Marine Sanctuary in Florida, then at the Caribbean Marine Research Center in the Bahamas. They carted tiny reef samples back to the field lab and examined them with a fluorometer, which assesses damage to photosynthesis. We now know that, if water temperature is too high for too long, everything goes wrong very quickly - like throwing a screwdriver into a running engine. In addition to excessively warm temperature, a number of other factors, including pollution, may be contributing to this coral bleaching.
Tropical corals are already on the edge of their 'temperature envelope' of life during most summers.