Coral Reefs

Submitted by Arthur Dahl on 5. June 2011 - 18:21
e-learning centre on sustainable development


Heading: Environment    Topic: Coral Reefs

The latest report on the status of coral reefs (Wilkinson, 2008) shows that reefs are continuing to decline where there are heavy local human impacts, but that overall there has been some improvement where more remote reefs are gradually recovering from the severe bleaching of 1998 (see below). Nineteen percent of reefs were considered totally degraded in 2008, down from 20% in 2004, while 15% are threatened with immediate collapse within 20 years and 20% in the longer term (20-40 years), totalling 54%. In 2004, this figure was 70%. The remaining 46% of reefs are not threatened by local impacts but could be vulnerable to climate change and ocean acidification.

Shifts in ocean conditions produced by the El Niño/Southern Oscillation, and rising temperatures due to global climate change, can bleach corals and other animals on tropical coral reefs. Corals usually live near the upper limit of their temperature tolerance, and even a small increase in temperatures above their normal temperature limit can stress them and lead to the expulsion of their symbiotic algae, which give them their colors as well as providing nourishment. Corals can often recover from short bleaching events, but extended or repeated bleaching can be fatal.

During the major El Niño in 1997-98, bleaching occurred on coral reefs ranging from Kenya to French Polynesia and Baja California in the Indo-Pacific, and from the Florida Keys to the Yucatan coast in the Caribbean. For instance, in the Maldives, a massive bleaching event was reported in May 1998. More than 90% of animals with algal symbionts, including corals, giant clams, anemones and soft corals showed heavy bleaching to depths of 20 metres at 8 locations on North Mahe atoll. Swimming up the reef was described as like going up the snowcapped Alps. Conditions at the time in the Maldives included surface water temperatures of 32 degrees C extending down to 15 metres, very low wind with no surface mixing, and a low tidal range of less than half a metre (Elder, 1998).

Bleaching was also reported early in 1998 as starting in the southernmost part of the Australian Great Barrier Reef and moving northward, reaching New Caledonia. Corals in the Galapagos Islands bleached at temperatures of 29 degrees C, a degree and a half warmer than the critical temperature for bleaching at that site. Sea surface temperatures in the major El Niño events of the 1980s were not quite this warm (Strong, 1998).

In the Caribbean on the Belize barrier reef, the largest in the Northern Hemisphere, sea surface temperatures, which rarely exceed 29 degrees C, reached 31.5 degrees and were greater than 30 degrees for months in 1998. This killed the most abundant coral on the reefs, Agaricia tenuifolia, causing the first recorded complete collapse of a coral population from bleaching in the Caribbean, and damaging other species, as demonstrated by surveys in 1999 and 2000. Coral cores from the reef showed that no similar bleaching had happened for more than 3,000 years (Aronson et al, 2000).

A new episode of bleaching was reported in the mid-Pacific from mid-February to late April 2000, associated with La Niña, the reverse of El Niño. Hot spots ranging from the Solomon Islands to Easter Island measured 30-31.5 degrees C caused bleaching in 50 to 90 percent of the corals. For instance, in Fiji, up to 90 percent of corals bleached down to a depth of 30 metres. All coral species were affected, although some shallow water populations showed selective recovery (ICRI, 2000).

In addition to the short-term threat represented by coral bleaching, coral reefs are under pressure from a variety of human impacts. The accelerating decline of coral reef ecosystems is described in the series of global assessments of the Status of Coral Reefs of the World (Wilkinson, 1998; 2000; 2004, 2008), based on information collected and updated by the Global Coral Reef Monitoring Network. They detail the continuing deterioration of coral reefs in all areas where human activities are concentrated, notably along the coast of eastern Africa, all of continental South Asia, throughout Southeast and East Asia and across the wider Caribbean region.

The extent of world-wide degradation of reefs was first documented by a global survey undertaken by Reef Check 1997 ( Many coral reef fisheries resources are being pushed towards extinction everywhere by heavy over-exploitation, and habitat degradation from land-based activities and runoff is widespread on inhabited coasts.

The first global assessment of coral reefs to map areas at risk from overfishing, coastal development and other human activity (Bryant et al., 1998), shows threats to 58 percent of the world's coral reefs. A third of all reefs are threatened by overexploitation, and a third also by coastal development. At least 11 percent are "hot spots" with high levels of reef fish biodiversity that are under high threat from human activities, including Philippine and Indonesian reefs, Tanzania and the Comoros along the East African coast, and the Lesser Antilles in the Caribbean. In Southeast Asia, more than 80 percent of the most species-rich coral reefs on earth are threatened by coastal development and fishing pressures, and over half are at high risk. Nearly two thirds of Caribbean reefs are in jeopardy, especially around islands like Jamaica, Barbados and Dominica. Pacific reefs are the least threatened, with only 40 percent at medium or high risk of damage. While there are over 400 marine parks and reserves with coral reefs, most are very small, and at least 40 countries have no protected areas for their coral reefs (

Another concern is the recent discovery that coral reefs are threatened by rising levels of carbon dioxide in the atmosphere, which may lower calcification rates of corals, coralline algae and coral-algal communities by up to 10-20 percent as carbon dioxide levels double in the next century. (Coral Reefs and Global Change,1998) This will make it harder for them to build their skeletons and add to the reef. Other estimates suggest that the increase in atmospheric CO2 will reduce carbonate ion concentration in the surface ocean by 30 percent and cause a potential reduction in coral growth over the next 65 years of 40 percent (Langdon, 2000).

More general information on the status of coral reefs is available from the NOAA Coral Health and Monitoring Project at


Aronson, Richard B., William F. Precht, Ian G. MacIntyre and Thaddeus J.T. Murdoch. 2000. Ecosystems: Coral bleach-out in Belize. Nature 405: 36. 4 May 2000.

Bryant, Dirk, Lauretta Burke, John McManus and Mark Spaulding. 1998. Reefs at Risk: A Map-Based Indicator of Threats to the World's Coral Reefs. WRI/ICLARM/WCMC/UNEP. World Resources Institute, Washington, D.C., 1998& 56 p. Web version:

Coral Reefs and Global Change, 1998.  "Coral Reefs and Global Change: Adaptation, Acclimation or Extinction?" Initial Report of a Symposium and Workshop, Boston, January 3-11, 1998.

Elder, Danny. 1998. (UNDP consultant, Maldives). Personal report to Earthwatch 14 May 1998.

ICRI. 2000. Coral bleaching alarms Pacific experts - links made to climate change. International Coral Reef Initiative, Press Release 25 May 2000.

Langdon, Christopher. 2000 (in press). Global Biogeochemical Cycles.

Strong, Al. 1998.  (NOAA Oceanographer) quoted in Sea Technology, March 1998, p. 9 and p. 70.

Wilkinson, Clive (ed.). 1998. Status of Coral Reefs of the World: 1998. Australian Institute of Marine Science, Townsville, Australia. Web version:

Wilkinson, Clive (ed.). 2000. Status of Coral Reefs of the World: 2000. Australian Institute of Marine Science, Townsville, Australia. 

Wilkinson, Clive (ed.) 2004. Status of Coral Reefs of the World 2004. Australian Institute of Marine Science, Townsville, Australia.

Wilkinson, Clive (ed.) 2008. Status of Coral Reefs of the World 2008. Australian Institute of Marine Science, Townsville, Australia.

Article last updated 1 March 2009

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