Climate change, coral bleaching and the future of the world's coral reefs

Sea temperatures in many tropical regions have increased by almost 1 degree s C over the past 100 years, and are currently increasing at similar to 1-2 degrees C per century. Coral bleaching occurs when the thermal tolerance o f corals and their photosynthetic symbionts (zooxanthellae) is exceeded. Ma ss coral bleaching has occurred in association with episodes of elevated se a temperatures over the past 20 years and involves the loss of the zooxanth ellae following chronic photoinhibition. Mass bleaching has resulted in sig nificant losses of live coral in many parts of the world. This paper consid ers the biochemical, physiological and ecological perspectives of coral ble aching. It also uses the outputs of four runs from three models of global c limate change which simulate changes in sea temperature and hence how the f requency and intensity of bleaching events will change over the next 100 ye ars. The results suggest that the thermal tolerances of reef-building coral s are likely to be exceeded every year within the next few decades. Events as severe as the 1998 event, the worst on record, are likely to become comm onplace within 20 years. Most information suggests that the capacity for ac climation by corals has already been exceeded, and that adaptation will be too slow to avert a decline in the quality of the world's reefs. The rapidi ty of the changes that are predicted indicates a major problem for tropical marine ecosystems and suggests that unrestrained warming cannot occur with out the loss and degradation of coral reefs on a global scale.