As more frequent bouts of severe heat stress plague the world’s oceans, coral reefs don’t have enough time to recuperate, a new report shows.By Teresa TomassoniIn 1998, unusually hot ocean temperatures stressed corals worldwide, causing them to expel the microalgae living within their tissues that normally nourish the tiny animals and give them color. Over the next year, more than 20 percent of the world’s reefs turned white during the first global coral bleaching event on record.