Reef building coral, like trees, lay down annual growth bands and are extremely sensitive to their immediate environment. Corals found in the equatorial region of the Pacific have been known to live up to 350 years in an energetic and well-mixed region and therefore often record conditions for a relatively large area. A coral's growth can be affected easily by changes - in upwhelling, winds, convection, and sea surface temperatures - causing distinct chemical and thermal changes as brought on by factors like El Niño. Chemical and isotopic records found in the skeletons of the hermatypic corals from the equatorial region of the Pacific can provide information on the effects of the variation of temperature to a particular environment in which they live, providing an excellent paleoecological record of past climate variations.

(Taken from coral.aoml.noaa.gov)