It seems that zooxanthellae start to have serious health problems after only a few days of higher seawater temperature. This increase may be only one or two degrees above their maximum tolerance. It is thought the zooxanthellae food making processes start producing chemicals (oxygen radicals) that are poisonous to the coral polyp.
Obviously the coral polyp knows when a friendship needs to come to an end, so the polyp starts to expel its zooxanthellae back into the sea. If the temperature remains high for about four weeks, the polyps will not contain enough zooxanthellae and will die. Otherwise the coral polyps will slowly recover, but it can take years for the reef to return to its former glory.