Climate Change, Coral Bleaching and the Future of the World's Coral Reefs

CLIMATE
CHANGE
CORAL
BLEACHING
and the
FUTURE
of the
WORLDÕS
CORAL
REEFS
by
O V E H O E G H Ð G U L D B E R G
ASSOCIATE PROFESSOR,
SCHOOL OF BIOLOGICAL SCIENCES,
UNIVERSITY OF SYDNEY
DIRECTOR,
THE CORAL REEF RESEARCH INSTITUTE,
UNIVERSITY OF SYDNEY
ISBN 90-73361-52-4
Sea temperatures in the tropics have increased by almost
1oC over the past 100 years and are currently increasing at the rate of approximately 1-2oC per century. Reefbuilding corals, which are central to healthy coral reefs, are currently living close to their upper thermal limit.
They become stressed if exposed to small slight increases
(1-2oC) in water temperature and experience coral bleaching. Coral bleaching occurs when the photosynthetic symbionts of corals (zooxanthellae) become increasing vulnerable to damage by light at higher than normal temperatures. The resulting damage leads to the expulsion of these important organisms from the coral host. Corals tend to die in great numbers immediately following coral bleaching events, which may stretch across thousands of square kilometers of ocean. Bleaching events in 1998, the worst on record, saw the complete loss of live coral from reefs in some parts of the world.
This paper reviews our understanding of coral bleaching and demonstrates that the current increase in the intensity and extent of coral bleaching is due to increasing sea temperature. Importantly, this paper uses the output from four different runs from two major global climate models to project how the frequency and intensity of bleaching events are likely to change over the next hundred years if greenhouse gas emissions are not reduced. The results of this analysis are startling and a matter of great concern.
Sea temperatures calculated by all model projections show that the thermal tolerances of reef-building corals are likely to be exceeded within the next few decades. As a result of these increases, bleaching events are set to

References: 60. Goenaga, C., Vicente, V., and Armstrong, R. (1988) Aposymbiosis in Puerto Rican zooxanthellate cnidarians 61. Goreau, T.J. and Hayes, R.M. (1994) Coral bleaching and ocean "Hot spots". 62. Goreau T.J., Hayes R.M., and Strong, A.E. (1997) Tracking south pacific coral reef bleaching by satellite and field observations 63. Goreau, T.J. (1964) Mass expulsion of zooxanthellae from Jamaican reef communities after hurricane Flora 64. Goreau, T. J. (1992) Bleaching and Reef Commumity Change in Jamaica: 1951 - 1991 65. Goreau, T. J., and MacFarlane, A. H. (1990) Reduced growth rate of Montastrea annularis following the 1987-1988 coral-bleaching event 66. Harriot, V.J. (1985) Mortality rates of scleractinian corals before and during a mass bleaching event 67. Harrison, PL and Wallace, CC. (1990) Reproduction, dispersal and recruitment of Scleractinian corals [Coral Reefs] 68. Hatcher, B.G. (1988) Coral Reef Primary Productivity: AbeggarÕs Banquet. 69. Hoegh-Guldberg, O. (1989) Regulatory Biology of Endosymbiosis Ph.D. 70. Hoegh-Guldberg, O. (1994) The population dynamics of symbiotic zooxanthellae in the coral Pocillopora damicornis exposed to elevated ammonia. 71. Hoegh-Guldberg, O. and Pearse, J. S. (1995) Temperature, food availability and the development of marine invertebrate larvae 72. Hoegh-Guldberg, O. (1995) The mass bleaching of coral reefs in the Central Pacific in 1994 73. Hoegh-Guldberg, O. (1994) Mass bleaching of coral reefs in French Polynesia, April 1994 74. Hoegh-Guldberg, O. (1999) The impact of increased concentrations of ammonium and phosphate on coral growth and survivorship under field 75. Hoegh-Guldberg, O., and Jones, R. (1999) Diurnal patterns of photoinhibition and photoprotection 76. Hoegh-Guldberg, O. and Salvat, B. (1995) Periodic mass bleaching of reef corals along the outer reef slope in Moorea, French Polynesia 77. Hoegh-Guldberg, O. and Smith, G.J. (1989) The effect of sudden changes in temperature, irradiance and salinity on the population density and export of zooxanthellae from the reef corals Stylophora pistillata (Esper 1797)and Seriatopora hystrix (Dana 1846) 78. Hoegh-Guldberg, O., Berkelmans, R. and Oliver, J. (1997) Coral bleaching: Implications for the Great Barrier Reef Marine Park 79. Houghton, J. T., Callander, B. A. and Varney, S. K. V. (eds) (1992) Climate Change 1992 80. Houghton J.J., Meiro Filho L.G., Callander B.A., et al. (eds) (1995) Climate change 1995: The science of climate change: contribution of working group I to 81. Hughes, T. P. (1989) Community structure and diversity of coral reefs: the role of history 82. Hughes, T. P. (1994) Catastrophes, phase shifts, and large-scale degradation of a Caribbean coral reef 83. Hughes, T. P. and Jackson, J. B. C. (1985) Population dynamics and life histories of foliaceous corals 84. Hughes, T.P., Reed, D.C., and Boyle, M. (1987) Herbivory on coral reefs: community structure following mass mortalities of sea urchins. 85. Hurrell, J. W., and Trenberth, K. E. (1997) Spurious trends in the satellite MSU temperature record arising from merging different satellite records 87. ICRS (Dec 1998) Statement on Global Coral Bleaching in 1997-1998, International Coral Reef Society, December 1998. 88. Iglesias-Prieto R. (1995) The effects of elevated temperature on the photosynthetic responses of symbiotic dinoflagellates 89. Iglesias-R., Matta, W.A., Robins, W.A. Trench, R.K.(1992) Photosynthetic response to elevated temperature in the symbiotic dinoflagellate Symbiodinium 90. Jameson, S. C., McManus, J. W., and Spalding, M. D. (1995) State of the Reefs: Regional and Global Perspectives 91. Jokiel, P. L. (1980) Solar ultraviolet radiation and coral reef epifauna. Science 207:1069 1071. 92. Jokiel, P.L., and Coles, S.L. (1990) Response of Hawaiian and other Indo- Pacific reef corals to elevated temperatures 93. Jokiel, P. L., and Coles, S. L. (1977) Effects of temperature on the mortality and growth of Hawaiian reef corals 94. Jokiel, P. L. and York, R. H. Jr. (1982) Solar ultraviolet photobiology of the reef coral Pocillopora damicornis and the symbiotic zooxanthellae 95. Jones, R.J. (1995) Sublethal stress assessment in scleractinia and the regulatory biology of the coral-algal symbiosis 96. Jones, R.J. (1997a) Zooxanthellae loss as a bioassay for assessing stress in corals 97. Jones, R.J. (1997b) Changes in zooxanthellar densities and chloro-phyll concentrations in corals during and after a bleaching event