Past ENSO Variability

This chapter investigates ENSO variability before the instrumental era. Though generally indirect, paleoclimate observations provide information that no other source can, sampling ENSO behavior across different base states, subject to many types and intensities of external forcing, and providing a much longer statistical sample than afforded by the instrumental record. After first reviewing the nature, strengths, and caveats of the paleoclimate observations most relevant to ENSO, we outline how these observations may be used to infer changes in ENSO properties over time. The chapter then synthesizes the most robust paleoclimate inferences about ENSO over various time intervals: the Pliocene, Quaternary Ice Ages, the Holocene, the last millennium, and the anthropogenic era. ENSO appears to have operated on Earth for at least 3 million years, and the existing observations support the view that variations in ENSO amplitude and frequency arise primarily from processes internal to the climate system. However, multiple lines of evidence support the notion that ENSO properties are sensitive to large changes in mean climate, such as those seen during the anthropogenic era. Throughout these examples, a case is made that paleoclimate observations are now mature enough to offer quantitative constraints on ENSO and its representation in climate models, offering a key out-of-sample test of model predictions across a variety of climate scenarios. The chapter closes with a roadmap for furthering the relevance of paleoclimate observations to the study of ENSO.