Coral bleaching is the term used for a loss of colour in reef-building corals and the subsequent visibility of the underlying (white) skeleton. It reflects damage to, or loss of, microscopic algae that live symbiotically within the coral tissue. Bleaching is caused by various types of stress, notably temperature extremes, but also pollution, solar radiation, fluctuating salinities, extremely low tides and exposure to air and often a combination of these factors. Coral reefs can survive if the bleaching episode is brief, but if prolonged, they die. It takes at least a decade for a full recovery of even the fastest growing corals; mass bleaching events 12 months apart offers zero prospect of recovery for reefs.
Coral bleaching is a complex phenomenon. Understanding the causes and consequences of coral bleaching events requires the knowledge, skills and technologies of a wide variety of disciplines. Any action aimed at addressing the issue should bear in mind the ecosystem approach, incorporating both the ecological and societal aspects of the problem.
Coral-bleaching events are a warning of even more severe impacts to marine systems. If anomalous sea-water temperatures continue to rise, become more frequent, or are prolonged, the physiological thresholds of other organisms will be surpassed. Not only will local fisheries be impacted, but certain high-value commercial pelagic fisheries and coastal ecosystems will be affected as well.
Trends of the past century suggest that coral bleaching events may become more frequent and severe as the climate continues to warm, exposing coral reefs to an increasingly hostile environment. Should seawater temperatures rise, either as a result of greenhouse gas emissions or natural variability in the ocean/atmosphere system, then we might expect the incidence and severity of coral bleaching to increase yet further in the future with the possibility of substantial changes to the coral reef community structure.
Nearly 60 per cent of the world's coral reefs are threatened by localized, human activities that have the potential to exacerbate the impacts of coral-bleaching events.
In the mid-1980s, coral bleaching began to occur on a large scale. In 1998, coral bleaching was more severe than ever before and occurred in at least 60 countries. Although the links between global climate change, El NiÃ±o phenomena and extensive coral bleaching are still subject to debate, it has been suggested that only global warming could have induced such extensive bleaching simultaneously throughout the disparate reef regions of the world. Evaluations of the 1998 coral-bleaching events suggest that marine protected areas alone may not provide adequate protection for at least some corals and other reef-associated species as sea-surface temperatures rise. They also suggest that coral-reef conservation can no longer be achieved without consideration of the global climate system and that it requires efforts to mitigate accelerated global climate change.
In 2002, an epidemic of coral bleaching has hit the Great Barrier Reef in Australia, the world's largest coral reef, for the second time in four years. It is also reported to be spreading through the coral islands of the South Pacific, including Tahiti, the Cook Islands, New Caledonia and Fiji.