The 3rd Global Coral Bleaching (GCBE3) event since 1997 is underway.
It started slowly in 2014 and will last through 2016 — 3 years. It has already bleached and killed large areas of coral reef in several parts of the world, including Hawaii, American Samoa and Florida. Based on NOAA’s Coral Watch Program (released in September 2015 and in parts shown below) the long range predictions are grim and this could be the worst event in history for parts of the Indo-Pacific.
Parts of the Caribbean, Persian Gulf, S China and southern Baja Calif are bleaching (or are predicted to be) now and the Caribbean will experience peak bleaching in October / November. During early to mid- 2016, parts of Australia (Great Barrier Reef) and Indonesia will be threatened with some of the hottest water in history. Because the NOAA predictions are often underestimates, it is possible that in a worst case scenario as much as 25% of the remaining reefs in the world could be lost over the next 12 months. The degree and extent of the damage could affect food security and economies of many developing countries; Small Island Developing States are particularly vulnerable. The immediate damage to tourism and fisheries could run into billions of dollars and could affect hundreds of millions of people.
What is coral bleaching?
When corals are stressed by changes in water conditions such as temperature, light, pollution or nutrients, they expel their symbiotic algae living on their tissues, causing them to loose their color and turn completely white.
Warmer water temperatures can result in coral bleaching. When the water is getting too warm, corals will expel the algae (zooxanthellae) living on them causing the coral to loose their colors and turn completely white. When a coral bleaches, it is not dead yet, but they are under more stress.
Can corals survive bleaching?
Yes they can, if the stress caused bleaching is not severe and for too long, coral can recover. If the stress continues and the algae loss is prolonged the coral will eventually die.
You can find more detailed information about coral bleaching on Wikipedia