Figures released to the Guardian by NOAA show that about 12% of the world's coral reefs have suffered bleaching in the last year. Perhaps as much as 12,000 sq km of coral may be lost forever. Bleaching occurs when coral polyps eject their symbiotic algae partners due to increased ocean temperatures. Corals can only tolerate a narrow range of temperatures. If water temperatures warm by just 1℃ for more than week the fatal ejection of photosynthesizing algae begins. The algae color the white calciferous coral skeletons. Global bleaching events have been recorded twice before in 1998 and 2010. The 1998 event wrought major devastation when a massive El Niño set a chain of events that killed between 16-19% of the world's coral reefs. Before these events bleaching of coral reefs has not occurred in thousands of years. The events represent a major change in the environmental conditions of Earth's tropical oceans. During the 1998 bleaching event Australia's Great Barrier Reef recorded the highest sea temperatures ever*.
returning to reef areas only now recovering from the last destruction. This mass of hot water is thought to be generated by global warming. Research into bleaching shows that while corals can recover from mild bleaching in a decade or more of regrowth, an extreme event kills coral reefs, and they are replaced by other organisms such as seaweed and algae. Such degradation occured in the Seychelles after the 1998 event. Global temperature has increased only 0.8℃. As the global temperature increases even more corals already stressed by higher water temperatures will recover more slowly if at all, increasing the percentage of dead coral reefs.
*I. McCalman, The Reef 2013, p.270