The narrow temperature range within which most corals are at their healthiest is very close to the upper lethal limit. Temperatures of only 1 or 2°C above the usual summer maximum can be lethal and can also trigger an increasingly commonly observed phenomenon known as coral bleaching. The beautiful colours of many corals and some of their relatives such as anemones and sea whips mostly come from the symbiotic zooxanthellae living within their tissues. Most or all of these algal cells may be expelled by the coral when it is under stress so that the white skeleton shows through. If the bleaching is severe or prolonged, the coral often dies. Shorter episodes may allow the coral to rebuild its algal population and survive.
Recent large-scale bleaching events on coral reefs seem to be related most consistently to higher-than-normal water temperatures. In 1982-3 there was an unusually severe El Niño southern oscillation event (see Section 1.3.6) which increased water temperature in the eastern Pacific 3 to 4°C above the usual average. Corals became bleached at many places throughout the Indo-Pacific. Up to 90 per cent of corals were killed on some reefs including those in Panama, Costa Rica and the Galapagos. Widespread bleaching occurred again between 1987 and 1988, another strong El Niño year. Some scientists link the apparently increasing severity and occurrence of El Niño to global climate change and suggest that bleaching is one of the first concrete manifestations of global warming. Considerably more detailed and standardized data on seawater temperatures and on the physiological responses of corals to temperature and stress are needed before this conclusion can be justified. However, the Intergovernmental Panel on Climate Change has predicted a 1 or 2°C temperature increase over the next 50 years, and if this proves correct, there is little doubt that the consequences for coral reefs could be disastrous.
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