El Nino is a state of the ocean-atmosphere system in the tropical Pacific that brings important consequences for weather around the globe. The El Nino current brings warm waters eastward from the western Pacific and leaves low tides in the western Pacific. Both can cause the bleaching and death of corals, damaging the balance of these ecosystems. NOAA is predicting a strong El Nino for this year. The El Nino event usually occurs every two to seven years.
Many corals live in water that is already near their upper temperature limit.
When bleached, corals lose the food that the algae normally provide. While corals often recover their zooxanthellae and survive through bleaching events, they often die if the stress is too severe or prolonged. In 1982-83 the largest El Nino on record reached the eastern Pacific and 70 to 95 percent of the corals in Costa Rica, Panama, Colombia and the Galapagos Islands died because of bleaching. The damage did not stop with the bleaching. The death of some species of corals removed protection against predators such as the Crown of Thorns starfish.