File:Partially Bleached Coral.jpg

From Global Warming Art


This image shows a partially bleached coral (species Pocillopora). Coral bleaching occurs when, as a result of stress, the coral expel or digest the photosynthetic zooxanthella algae that ordinarily form a symbiotic relationship with the coral. As the zooxanthella provide the coloring to the coral, a bleached coral appears white because that is the color of the underlying calcite skeleton. As each coral is actually a colony made up of many discrete organisms, known as coral polyps, it is possible under moderate stress that only portions of the coral will undergo bleaching, such as shown here.

The cause and biological utility of coral bleaching is not well understood, but it is clear that it is triggered by environmental changes, including changes in ocean temperature or salinity. A bleached coral can survive for up to several months, but will ultimately die if it is not repopulated with zooxanthella. It is hypothesized that the bleaching may be an adaptive response of the coral itself to environmental stress by removing zooxanthella that are poorly adapted to the changed environment so that they may be replaced by other zooxanthella that are better adapted to the changes. Observations have shown that when a bleached coral is repopulated with zooxanthella they often come from a different selection of species than the ones that were lost.[1] However, the coral will die if no zooxanthella are able to establish themselves under the conditions that the coral finds itself after bleaching.

Since ~1979, coral bleaching events have become more common as a result of increasing ocean temperatures associated with global warming, with the most severe event on record occurring during the exceptional warmth and El Nino conditions of 1998. In this context, coral bleaching is often associated with temperatures at least 1 °C above the long-term summer maximum.[2]. The 1998 event severely impacted or killed robust coral colonies that may have persisted more than 700 years[3], providing evidence of the unusual extent of recent bleaching. Predictions for future temperature increases are such that bleaching events could occur every summer for most coral reefs before 2050.[2]


This image is taken from the Pacific Islands Fisheries Science Center of the National Oceanic and Atmospheric Administration.

Public domain

This image is a work of the National Oceanic and Atmospheric Administration, taken or made during the course of an employee's official duties. As works of the U.S. federal government, all NOAA images are in the public domain.


  1. ^ W. W. Toller, R. Rowan, and N. Knowlton (2001). "Repopulation of Zooxanthellae in the Caribbean Corals Montastraea annularis and M. faveolata following Experimental and Disease-Associated Bleaching". The Biological Bulletin 201: 360-373. 
  2. ^ a b Ove Hoegh-Guldberg. "Climate change, coral bleaching and the future of the world's coral reefs". Marine and Freshwater Research: 839-866. 
  3. ^ International Society for Reef Studies (1998). ISRS Statement on Global Coral Bleaching in 1997-1998.

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