Global climate change and the thermal tolerance of corals:
Implications for the 21st century
O. Hoegh-Guldberg
Centre for Marine Studies
University of Queensland
St Lucia, Qld 4072, Australia
Elevated sea temperatures (above seasonal summer maxima) have been the primary cause of the major episodes of coral bleaching since 1979. Reef-building corals and other symbiotic invertebrates tend to die in greater numbers after severe episodes of coral bleaching like that of 1998. Tropical sea temperatures have increased by almost 1℃ over the past 100 years and are currently increasing at 1-2℃ per century around the globe. Japan's coral reef waters are no exception and have increased by 1.4℃ since 1930 (Okinawa) and are currently increasing at the rate of 2.3℃ per century (95% confidence interval, 1.4 - 3.1℃). Together with the explicit link between coral bleaching, mortality and water temperature, rising sea temperatures have generated concern that coral bleaching may become a regular (annual) feature of coral reef environments in the 21st century. The central question is, then: How will coral reefs respond to an increased frequency and intensity of coral bleaching and associated mortality? The answer to this question in turn comes down to two fundamental issues. These are: (a) Adaptation. How fast will susceptible organisms (i.e. Corals, zooxanthellae and other related symbioses) evolve to higher sea temperatures?, and (b) Genetic diversity and connectivity. Are reefs genetically connected enough such that corals and zooxanthellae adapted over evolutionary time to warmer, lower latitudes can rapidly migrate to areas in which less adapted corals have been eliminated? The amount of work in these areas is limited and must be a priority of future research. Underpinning both of these concepts is the notion of time scale. The observation that corals have adapted to changes in sea temperature (or to catastrophic climate events) in the past does not counter the idea that apparent changes in sea temperature represent a problem for global reef health. Adaptive responses to these changes are likely to have taken hundreds if not thousands of years, and certainly did not occur over decades. The implications for these changes are serious. If the function and health of the world's coral reef ecosystems is interrupted for decades by the expected changes in sea temperature, quite substantial and negative ramifications for coral reef dependent societies are likely to result.
