Global climate change and the thermal tolerance of corals

The thermal environment of reef-building corals is critical to their distribution and survival. Recent evidence has revealed that the mean global temperature has increased by 1 degree Celsius over the past century. Examination of tropical sea temperatures reveal that they have also increased by almost 1°C over the past 100 years and are currently increasing at the rate of approximately 1-2°C per century. Japanese waters are no exception. Waters off Okinawa have increased by 1.4°C since 1930 and are currently increasing at the rate of 2.3°C per century (95% confidence interval, 1.4-3.1°C). Coral bleaching occurs when the thermal tolerance of corals and their zooxanthellae are exceeded, probably due to an increased sensitivity of the zooxanthellae to chronic photoinhibition. The consequences of bleaching can be devastating, with corals dying in vast numbers and such aspects as coral reproduction being severely curtailed. In 1998, the world's coral reefs experienced the worst bleaching on record. Japanese reefs were severely effected, with bleaching coinciding with a massive temperature anomaly in July-August 1998. Vast numbers of corals died. The intention of this paper is collect what we know about coral bleaching from biochemical, physiological and ecological perspectives and to discuss to how reefs (especially around Okinawa) might change in the next century. A single important issue surfaces in these deliberations. If temperatures continue to increase, then corals will experience greater incidences of bleaching and mortality unless they can acclimate physiologically or adapt genetically. The conclusions that stem from this issue are of great concern. Firstly, available evidence suggests that corals are not acclimating physiologically to any really extent to the sporadic and seasonal changes in sea temperature. Secondly, the rate of change in sea temperature arguably exceeds the capacity of coral populations to genetically adapt fast enough. The third conclusion is the most worrying. If corals are not changing fast enough, then coral reefs will experience more frequent and more intense bleaching. Given the outcome for many coral reefs across the planet during the 1998 episodes of bleaching, this has to be of great concern to coral reef users and managers everywhere.