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Early Warning Signs of Global Warming: Coral Reef Bleaching

Coral reefs are one of the most productive ecosystems on Earth, providing many critical services to fisheries, shoreline protection, tourism, and to medicine. They are also believed to be among the most sensitive ecosystems to long-term climate change (Nurse et al., 1998). Elevated sea surface temperatures can cause coral to lose their symbiotic algae, which are essential for the nutrition and color of corals. When the algae die, corals appear white and are referred to as "bleached." Water temperatures of as little as one degree Celsius above normal summer maxima, lasting for at least two to three days, can be used as a predictor of coral bleaching events (Goreau and Hayes, 1994). Studies indicate that most coral are likely to recover from bleaching if the temperature anomalies persist for less than a month, but the stress from sustained high temperatures can cause physiological damage that may be irreversible (Wilkinson et al., 1999).

In 1998 coral reefs around the world experienced the most extensive and severe bleaching in recorded history (ISRS, 1998; Wilkinson et al., 1999). Coral bleaching was reported in 60 countries and island nations at sites in the Pacific Ocean, Indian Ocean, Red Sea, Persian Gulf, Mediterranean and Caribbean. Indian Ocean corals were particularly severely impacted, with greater than 70 percent mortality reported in the Maldives, Andamans, Lakshadweep Islands, and in Seychelles Marine Park System. Unlike most previous bleaching events in which severe impacts were limited to less than 15 m water depth the 1998 bleaching affected corals at up to 50 m water depth. This mass bleaching followed similar but less severe events in 1987 and 1990. Prior to the early to mid 1980s, bleaching tended to be rare and localized, and corals generally recovered.

The 1998 mass bleaching was coincident with anomalously high sea surface temperatures. That year was the warmest of this century (NOAA, 1999), and tropical sea surface temperatures were the highest in the modern record (Strong et al., 1998). For many parts of the Pacific, the 1997-98 mass bleaching has been linked to the strong El Niño-induced seawater warming. The relationship between El Niño and coral bleaching is less clear, however, for the Indian Ocean, Arabian Gulf, and some parts of the Pacific (Wilkinson, 1999; ISRS, 1998). The geographic extent, regional severity, and increasing frequency of recent mass bleaching events point to an underlying global cause namely a trend of increasing sea surface temperatures in some of the tropical oceans, driven by global warming (US Department of State, 1999).

Global mean sea-surface temperatures are projected to increase by about 1-2°C in the next century (Kattenberg et al., 1996). If the overall warming is accompanied by more frequent periods of sustained high temperatures, mass bleaching events will become more frequent and widespread. Increasing human stresses such as pollution, overfishing, soil erosion, and physical damage from boats and other recreational activities will also weaken corals, limiting their ability to adapt to climate change (Hodgson, 1999; Nurse et al., 1998). Furthermore, as ocean warming coincides with sea-level rise and perhaps more frequent tropical storms and El Niños (e.g., Timmerman et al., 1999), reefs are likely to experience greater coastal erosion, sedimentation, and turbidity, which would add to their demise.

References

Goreau, T. J. and R. L. Hayes, 1994. Coral bleaching and ocean "hot spots." Ambio 23, 176-180.

Hodgson, G., 1999. A global assessment of human effects on coral reefs. Marine Pollution Bulletin 38, 345-355.

ISRS, 1998. ISRS statement on global coral bleaching in 1997-98. International Society for Reef Studies. http://www.uncwil.edu/isrs/

Kattenberg, A., F. Giorgi, H. Grassl, G. A. Meehl, J. F. B. Mitchell, R. J. Stouffer, T. Tokioka, A. J. Weaver, and T. M.L. Wigley, 1996. Climate models - projections of future climate, in Climate Change 1995: The Science of Climate Change, 285-357, (Eds J. T. Houghton, L. G. M. Filho, B. A. Callander, N. Harris, A. Kattenberg, and K. Maskell) Cambridge University Press, Cambridge, UK.

NOAA. 1999. Climate of 1998 annual review: global temperatures. National Climate Data Center. 12 January 1999. http://www.ncdc.noaa.gov/ol/climate/research/1998/ ann/global_temperatures.html

Nurse, L.A., R. F. McLean, and A.G. Suarez, 1998. Small island states. In The Regional Impacts of Climate Change: An Assessment of Vulnerability, 331-354, (Eds RT Watson, MC Zinyowera, RH Moss), Cambridge University Press, Cambridge, UK.

Strong, A. E., T. J. Goreau and R. L. Hayes. 1998. Ocean Hot Spots and coral reef bleaching: January July 1998. Reef Encounters 24, 20-22.

US Department of State, 1999. Coral bleaching, coral mortality, and global climate change. Report to the U.S. Coral Reef Task Force. Bureau of Oceans and International Environmental and Scientific Affairs, 5 March 1999. http://www.state.gov/www/global/ global_issues/coral_reefs/990305_coralreef_rpt.html

Timmerman, A., J. Oberhuber, A. Bacher, M. Esch, M. Latif and E. Roeckner, 1999. Increased El Nino frequency in a climate model forced by future greenhouse warming. Nature 398, 694-696.

Wilkinson C, O. Linden, H. Cesar, G. Hodgson, J. Rubens, and A.E. Strong, 1999. Ecological and socioeconomic impacts of 1998 coral mortality in the Indian Ocean: An ENSO impact and a warning of future change? Ambio 28, 188-196.

Additional Resources

An Introduction to Coral Reefs - An introduction to coral reef ecology by the University of the Virgin Islands. http://www.uvi.edu/coral.reefer/index.html

Brown, B.E., Odgen, J.C. 1993. Coral bleaching. Scientific American 269, 64-70.

Coral Bleaching Hotspots NOAA uses satellite observations of sea surface temperature to predict potential coral bleaching hotspots. Individuals can also report current or past incidents of coral bleaching using an electronic form. http://www.osdpd.noaa.gov/PSB/EPS/SST/climohot.html

Status of Coral Reefs of the world: 1998 Electronic version of a report on the status of coral reefs in 1998, including a detailed chapter on the 1997-98 bleaching event. http://www.aims.gov.au/pages/research/coral-bleaching/scr1998/scr-00.html

NOAA s Coral Health and Monitoring Program A comprehensive website with coral-related web links, access to literature lists, electronic listservs, and other resources. http://coral.aoml.noaa.gov/

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