Coral Bleaching

Coral reefs are one of the most biodiversed habitats on Earth. Reefs provide home to more than 25% of all marine species. They protect and support the lives of millions of people around the tropical zones, and are a font of wealth from fishing and recreation. Corals around the world are found to be severely threatened by the natural predators, coastal pollution, warming seas and human exploitation. The damage to reefs by warming seas is one of the most serious effects of global warming.

The most recent status of coral reefs report shows that 20% of the world’s coral reefs have been effectively destroyed and show no immediate prospects of recovery. Some 24% of the world’s reefs are under imminent risk of collapse through human pressures and a further 26% are under a longer term threat of collapse.

Coral bleaching results when sea temperature rises above a threshold temperature of about 28^oC. Temperatures higher than this will cause corals to expel colorful symbiotic algae (hence the bleaching). Deprived of metabolic by-products generated by algae for extended periods, corals die.

Coral bleaching was particularly pronounced during 1997-98 during a very strong El Niño year superimposed with a warm sea surface temperature, which scientist opinioned to be linked to global warming.

Australian scientists have reported an increase in coral disease on surveyed reefs in recent years, while coral disease has had a major impact on Caribbean reefs where it has destroyed 80% of coral in the past 20 years. Scientists believe that the increase in disease in some areas may be linked to higher sea temperatures.

Thermal expansion of sea water due to global warming has caused a rising rate of sea water of more than 8" a century, with a projected rise to a meter by 2100. Coral, which thrives near the sea surface may not respond well to this increase in water depth. In addition, seas are dissolving more and more carbon dioxide. Even though this adds more carbon, a raw material for coral making calcium carbonate reefs, it also acidifies the water, actually inhibiting the growth of coral.

The black band disease is seen here progressing across a colony of
favia speciosa on the Great Barrier Reef (Photo: Cathie Page)

Scientists analyzed the (calcification) growth rates of 328 coral colonies on 69 individual reefs that make up the 1,250 mile-long Great Barrier Reef, off north-east Australia. Data shows that the coral growth suffered a "severe and sudden" slowdown by 14.2% between
1990 - 2005, that is unprecedented in the last four centuries, according to scientists.

The same effects in both near-shore and offshore reefs were detected, suggesting it is not due to pollution from the land. "This study has provided the first really rigorous snapshot of how calcification might be changing," marine biologist Ove Hoegh-Guldberg of the University of Queensland in Australia told Science. "The results are extremely worrying."

References and related news:

El Nino and Coral Bleaching: NOAA
Slowdown of Coral Growth Extremely Worrying: Guardian/2009/Jan/01
Dead Fish, Coral Bleaching Raise Concerns about Papua New Guinea's ...

Experts: Climate Change Causes Coral Bleaching: Mystateline.com - Feb 16, 2009

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