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ENSO: A Permanent Feature of a Future Warmer World?
Reference
Davies, A., Kemp, A.E.S., Weedon, G.P. and Barron, J.A. 2012. El Niño-Southern Oscillation variability from the Late Cretaceous Marca Shale of California. Geology 40: 15-18.

Background
The authors write that "variations in the frequency and amplitude of the El Niño-Southern Oscillation (ENSO) recorded in both instrumental and paleoclimate archives have led to speculation that global warming may cause fundamental changes in this preeminent mode of global interannual climate variability (Fedorov and Philander, 2000)." More specifically, they state that there is speculation that "warmer climates may promote a permanent El Niño state (Wara et al., 2005; Fedorov et al., 2006)."

What was done
In a study designed to further explore this possibility, Davies et al. analyzed the latest Cretaceous laminated Marca Shale of California, which permits, as they describe it, "a seasonal-scale reconstruction of water column flux events and, hence, interannual paleoclimate variability," during what is known to have been a "past 'greenhouse' climate state."

What was learned
The four researchers report that "significant spectral peaks obtained from lamina-derived time series analysis of the Marca Shale closely resemble those of modern and historical ENSO variability." In addition, they indicate that "the parameters from which the time series are derived (biogenic- and terrigenous-lamina thickness and bioturbation index) appear directly related to the marine production and flux, incursion of oxygenated waters, and input of terrigenous sediment that would be influenced by ENSO-type mechanisms of interannual variability."

What it means
In light of their recent findings, Davies et al. say there is "little support for the existence of a 'permanent El Niño' in the Late Cretaceous, in the sense of the continual El Niño state depicted by Fedorov et al. (2006)," and they say this evidence "builds on results from the Cretaceous Arctic (Davies et al., 2011) and from younger Eocene and Miocene warm periods (Huber and Caballero, 2003; Galeotti et al., 2010; Lenz et al., 2010) to emphasize that there was robust ENSO variability in past 'greenhouse' episodes and that future warming will be unlikely to promote a permanent El Niño state," which point they also emphasize in the final sentence of their abstract, where they say that their evidence for robust Late Cretaceous ENSO variability "does not support the theory of a 'permanent El Niño,' in the sense of a continual El Niño-like state, in periods of warmer climate."

References
Davies, A., Kemp, A.E.S. and Palike, H. 2011. Tropical ocean-atmosphere controls on inter-annual climate variability in the Cretaceous Arctic. Geophysical Research Letters 38: 10.1029/2010GL046151.

Fedorov, A.V., Dekens, P.S., McCarthy, M., Ravelo, A.C., deMenocal, P.B., Barreiro, M., Pacanowski, R.C. and Philander, S.G. 2006. The Pliocene paradox (mechanisms for a permanent El Niño). Science 312: 1485-1489.

Fedorov, A.V. and Philander, S.G. 2000. Is El Niño changing? Science 288: 1997-2002.

Galeotti, S., von der Heydt, A., Huber, M., Bice, D., Dijkstra, H., Jilbert, T., Lanci, L. and Reichart, G.J. 2010. Evidence for active El Niño Southern Oscillation variability in the Late Miocene greenhouse climate. Geology 38: 419-421.

Huber, M. and Caballero, R. 2003. Eocene El Niño: Evidence for robust tropical dynamics in the "hothouse." Science 299: 877-881.

Lenz, O.K., Wilde, V., Riegel, W. and Harms, F.J. 2010. A 600 k.y. record of El Niño-Southern Oscillation (ENSO): Evidence for persisting teleconnections during the Middle Eocene greenhouse climate of Central Europe. Geology 38: 627-630.

Wara, M.W., Ravelo, A.C. and Delaney, M.L. 2005. Permanent El Niño-like conditions during the Pliocene warm period. Science 309: 758-761.

Reviewed 9 May 2012