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Reefs at risk

Global climate change and coral bleaching

Global climate change stands to alter sea-surface temperature, sea levels, oceanic currents and patterns of marine productivity. Coral reefs are particularly sensitive to climatic influences, exhibiting the phenomenon known as coral bleaching when stressed by higher sea temperatures and other factors. Coral bleaching, the paling or whitening of reef-building corals, results when coral polyps lose the microscopic algae, or zooxanthellae, living symbiotically within their tissues. Reef-building corals are highly dependent on this symbiotic relationship with zooxanthellae and frequently demise if the bleaching is prolonged. The corals may regain their symbiotic algae if the bleaching is not too severe, or if the environmental stressors causing it decrease in time.

This reaction of corals has been observed for many years, but reports of coral bleaching have been increasing greatly in recent years. Various types of stress, including temperature extremes, pollution and exposure to air, cause bleaching; but recent increases in temperature-related stress due to climate change are suspected in the increase in regional bleaching events. Recent reports have shown that bleaching events related to periodic climatic events remain the primary threat to coral reefs on the global scale. The extreme increase in sea-surface temperature associated with the major El Niño and La Niña climate switches in 1997-1998 resulted in extensive coral bleaching and mortality over large portions of the Indian Ocean and Southeast and East Asia. On some reefs, there were mortality levels greater than 90% leaving some reefs almost bare of corals and with early indication of major shifts in the population structures.

The critical features of recent coral-bleaching events are that areas have been struck indiscriminately, irrespective of the existing status of reef health. Impacts have been felt both on pristine remote reefs, and on reefs already under major human-induced stresses. Although some changes caused by bleaching events are not necessarily permanent, additional stresses, such as those created by pollution and physical degradation, exacerbate the effects these events and limit the recovery capability of coral reef ecosystems.