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        The World Conservation Monitoring Centre provides information services on conservation and sustainable use of the world's living resources, and helps others to develop information systems of their own.

         


        The 1998 Global Coral Bleaching Incident

        1998 is proving to be the worst on record for the phenomenon of coral bleaching, with shocking reports arising from countries across all of the world’s seas and oceans. Vast areas of coral reefs have been devastated, with up to 90% of corals dead or dying in many areas.

        What is coral bleaching?

        Corals, along with a number of other animals which inhabit coral reefs, live in a close association with a microscopic algae (zooxanthellae). These algae actually live inside the coral’s tissues in a partnership which benefits both organisms. Corals provide shelter and metabolic waste-products for the algae, while the algae provide many of the sugars and other nutrients which the corals need to survive. It is actually these same algae which add colour to corals and create the fabulous array of colours we associate with coral reefs.

        For reasons which are still unclear, this partnership breaks down when corals are stressed. The corals loose the algae from their tissues. At this stage the corals are not dead, but they become a vivid white colour, as if they had been bleached. The corals frequently recover from such an event, however this is by no means assured. If the cause of stress reaches particularly high levels, or remains for a long time-period the corals may die.

        A number of different stresses have been reported to cause coral bleaching, including high and low temperatures, pollution, unusually high or low light levels. By far the most common cause appears to be high temperatures. Corals are finely tuned to their environments and even relatively small deviations from environmental norms appear to cause stress. Temperatures even only 1 or 2 degrees Celsius higher than the mean averages at the warmest time of year are sufficient to cause the corals to bleach.

        What has happened in 1998?

        From the latter end of 1997 reports have been coming in of extensive coral bleaching from locations right across the globe.


        Map showing reported incidents of coral bleaching in the 1997/1998 period (various sources).

        To date reports have come in from over 32 countries, from all seas and oceans: Pacific, Atlantic, Indian Ocean, Red Sea, Caribbean, Arabian Gulf and even the Mediterranean (where there are some corals, but no true reefs). Without doubt this is the most geographically widespread bleaching event ever recorded. It is not completely ubiquitous, but we have to assume that bleaching has been more widespread than simply the areas where the reports have come in.

        In addition to the geographic spread of these events it is already apparent that the impacts have been more severe than ever previously recorded in many areas. Actual coral death is high, reaching 95% in some locations. In a few places very massive, centuries-old, corals have died. By contrast, it is important to note that in some places there has now been at least a partial recovery, with loss of only a few corals.

        The major cause of this bleaching is higher than average water temperatures which have been linked to one of the largest ever El Niño events of this century. The linkage of El Niño itself to human induced climate change is still somewhat unclear, but we can no longer doubt that natural extreme events (such as El Niño) are being compounded by human impacts. Thus, background rises in ocean temperatures will exacerbate El Niño effects. Other stresses such as pollution and sedimentation may likewise compound the changes induced by climate-change.

        How will this impact the world?

        Coral reefs are hugely important resources for millions, providing a key food supply, protecting coasts from erosion, alleviating poverty and providing a base for present and future for economic development, particularly from fisheries and tourism.

        Scientist are still very unclear as to the "knock-on" impacts of these events. The death of corals may well have an immediate impact on diving tourism revenues in some areas. The impacts on fish communities are far less clear, although it seems inevitable that some changes will occur: some fish will suffer the loss of corals, others may benefit from the increased amounts of algae. Over the longer term, if recovery is not quick it seems quite possible that the physical structure of the reefs themselves may be damaged, through the gradual impact of eroding organisms, or the dramatic changes driven by storms or destructive fishing practises. Loss of the reef structure will have considerable impacts on fisheries, diver tourism, coastal protection and even the shape and structure of coastlines themselves.

        Will the reefs recover?

        Reef corals have been present in the world for over 200 million years and are unlikely to disappear. We are probably not seeing the demise of coral reefs per se, we may be witnessing a significant decline in the general condition of a significant proportion of the world’s reefs. With a return to more ambient environmental conditions, recovery will undoubtedly occur, although it seems likely that the worst affected reefs could take decades to recover. The more worrying consideration is how already stressed reefs will recover, given the continuing pressures arising from overfishing, pollution and sedimentation. Over and above these is the consideration of how climate change will affect recovery. The world’s oceans are already slowly warming and it seems highly likely that increases of 1-2 degrees Celsius will occur by the middle of next century. Such changes will underlie any future El Niño events, so we have to ask whether reefs will indeed be given a chance to recover before future events of this sort will occur.

        What can we do?

        1) This problem is ecosystem-wide and ultimately the only solution is to cut greenhouse gas emissions, however there is some evidence that the impacts are exacerbated by additional stresses, such as pollution.

        2) Thus there remains the urgent need for better management of coral reefs and support for community based projects to reduce such stresses and support sustainable utilisation of resources. Such actions can no longer be limited to a small number of text-book examples, but must be widespread, in all countries with reefs.

        3) By no means all corals will have died in this event, and the remainder (which we may hope have some natural resilience to high temperatures) will provide new larvae which may be able to support recovery, even in more distant reefs (coral larvae are swept large distances by ocean currents). The establishment of a global network of well protected reefs which may be able to act as "source" reefs has never been more important.

        View Photograph (148k): Snappers swimming over bleached corals in the southern Seychelles, April, 1998.


        For further information please write to:
        Information Office,
        World Conservation Monitoring Centre,
        219 Huntingdon Road,
        Cambridge CB3 0DL,
        United Kingdom.
        Tel: +44 1223 277314;
        Fax: +44 1223 277136.
        Email: info@wcmc.org.uk
        Document URL: http:// www.wcmc.org.uk /latenews/bleaching.htm
        Revision date: 4-December-1998
        Current date: 21-January-2000

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