A fire coral in Bermuda. The one on the left is a healthy fire coral, while the one on the right is completely bleached. [Photo/XL Catlin Seaview Survey]

Coral bleaching occurs when corals are exposed to stressful environmental conditions such as high temperature. Corals expel the symbiotic algae living in their tissues, causing corals to turn white or pale. Without the algae, the coral loses its major source of food and is more susceptible to disease.

The first global bleaching event was in 1998, during a strong El Niño that was followed by an equally very strong La Niña. By the end of 1998, an estimated 16 percent of corals had been eliminated worldwide. A second one occurred in 2010.

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• Coral bleaching threat increasing in W. Atlantic, Pacific oceans

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