First, ocean warming is directly reducing coral cover through coral bleaching. Reef-building corals contain plant-like organisms called zooxanthellae that live symbiotically within their tissue. Zooxanthellae provide their coral host with food and oxygen and in return, the zooxanthellae receive nutrients, carbon dioxide, and an enemy-free shelter. This symbiotic relationship evolved tens of millions of years ago and has been critical to the success and evolutionary radiation of corals and to the development of reef ecosystems. When summertime water temperatures are just a degree or two warmer than usual for a few weeks, this critical yet delicate symbiotic relationship breaks down and the zooxanthellae are expelled, often leading to the coral’s death. (The greater the magnitude or duration of the warming, the greater the mortality and effect on coral populations.) The phenomenon is called “coral bleaching” because the coral animal appears to turn white after the zooxanthellae loss. This is because without their zooxanthellae symbionts, which contain various photosynthetic pigments, corals are nearly transparent and the white, external calcium carbonate skeleton that the coral polyps live on becomes plainly visible.
Coral reefs and tropical marine ecosystems are facing increasing stress from a variety of causes including over-exploitation, land-based sources of marine pollution and severe events including storms and tsunamis. They are also highly vulnerable to climate change, with 16% of the world’s reefs suffering serious damage during the global bleaching event of 1998. Climate Change is now recognized as one of the most serious long-term threats to the biodiversity and services provided by tropical marine ecosystems. The effects of climate change are visible today and predictions for coral reefs are extremely dire with many experts predicting the functional extinction of many coral reef systems during this century.
Globally, the economic and social importance of coral reefs has been acknowledged and it has been recognized as the first ecosystem to face 'irreversible changes' under the global temperature increases that are expected. Because of the great socio-economic importance of coral reefs, especially to the tourism and fisheries industries, the loss of these ecosystems would perturb the livelihoods of hundreds of millions of people around the world. As a result, the coral reef management community is engaging with the need for greater understanding, improved monitoring, management, and public awareness of the growing threat of climate change to coral reefs and dependant communities and sectors.