Dying coral bleached by global warming, Bahamas.

Credits: © Marine Photobank / Wolcott Henry 2001

Ocean warming is extremely dangerous to coral organisms, which are very sensitive to changes in temperature. Increased water temperatures, which may be linked to man-made global warming, can cause mass coral bleaching. Dissolved carbon dioxide also damages corals.

Corals grow in the warm waters, but many of them are near the limits of their tolerance for high temperatures. Bleaching is a breakdown of a "complex biological system" that corals have evolved in order to survive. Each coral formation is a colony of hundreds or thousands of tiny organisms (known as polyps) that jointly build a skeleton that forms the reef. The outside layer of each coral polyp is inhabited by tiny one-celled plants scientists called zooxanthellae. It is these organisms that give the coral its bright colours, and when they are expelled due to warmer water or some other stress, coral appears bleached (that is, go pale or snowy-white). Without zooxanthellae, the coral cannot survive for long.

Coral bleaching can be traced as far back as 1870. However, since the 1980s, bleaching events have become more frequent, widespread and severe. "The massive coral bleaching and mortality event of 1998 devastated large parts coral reefs around the world," says the World Resources Institute (WRI). The most affected reefs were in the Arabian/Persian Gulf, Kenya, Tanzania, the Seychelles, Maldives, Chagos banks, Sri Lanka and India in the wider Indian Ocean, parts of Southeast Asia, especially Vietnam, the Philippines, Taiwan, southern Japan, and Palau. Many areas reported coral losses of 60-90 per cent over large areas and often down to 30 metres or more. A report released by Greenpeace cites "warmer than normal temperatures" as the principal cause of the massive coral bleaching and mortality event. "Increased sea temperature is the primary reason why coral bleaching has occurred with increasing intensity and frequency over the past two decades," the report states.

Current estimates note that 10 per cent of all coral reefs are degraded beyond recovery. Thirty per cent are in critical condition and may die within 10 to 20 years. Experts predict that if current pressures are allowed to continue unabated, 60 per cent of the world's coral reefs may die completely by 2050.

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