A broad range of paleoclimate indicators is available to study the El Niño/Southern Oscillation (ENSO) phenomenon. These climate proxy records, each of which is sensitive to somewhat different aspects of climatic variations, can be used for studying ENSO – to answer specific questions, such as, How long has ENSO been operating in its present form? In this chapter, we evaluate the information provided by the various proxy records, highlighting their possible strengths and weaknesses, and pointing out areas where outstanding research questions into the nature of ENSO variability still remain.
At present, the preponderance of evidence suggests that during glacial times, the ENSO phenomenon (if it was operating as an alternating east-west source of heat and moisture to the atmosphere) did not leave the same spatial or temporal expression in the paleoclimate record that is evident more recently. Conditions in the early Holocene, prior to ~6,000 years before present (BP), are indicative of changed atmospheric and oceanic patterns, substantially different from those of today. Only after about 6,000 years ago did the climatic associations related to the changes in sea surface temperature and atmospheric circulation patterns related to ENSO that are seen today begin to be systematically recorded in the paleoclimate record.
One of the major expressions of global interannual climate variability is related to the coupled ocean-atmosphere phenomenon of El Niño/Southern Oscillation (ENSO). Because of the far-reaching effects of ENSO on societies, concerns about its future behavior have promoted research into the ENSO phenomenon's long-term history, especially of those intervals when climate boundary conditions (orbitally determined seasonal insolation, global ice cover, sea level, aerosols, etc.) were markedly different from those of today (Diaz and Markgraf 1992).