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Medieval Drought in Peru (and Elsewhere): What Does It Tell Us about El Niņos and Medieval Temperatures?
Reference
Rein B., Luckge, A. and Sirocko, F.  2004.  A major Holocene ENSO anomaly during the Medieval period.  Geophysical Research Letters 31: 10.1029/2004GL020161.

Background
Rein et al. state that "interannual climate variability along and off coastal Peru is dominated by ENSO," noting that in the hyperarid coastal deserts, heavy winter rainfalls only occur during times of maximum El Niņo strength (Philander, 1990).

What was done
The authors derived a high-resolution flood record of the entire Holocene from an analysis of the sediments in a 20-meter core retrieved from a sheltered basin situated on the edge of the Peruvian shelf about 80 km west of Lima, Peru.

What was learned
Rein et al. found a major Holocene anomaly in the flux of lithic components from the continent onto the Peruvian shelf during the late Medieval period.  Specifically, they report that "lithic concentrations were very low for about 450 years during the Medieval climatic anomaly from A.D. 800 to 1250."  In fact, they state that "all known terrestrial deposits of El Niņo mega-floods (Magillian and Goldstein, 2001; Wells, 1990) precede or follow the medieval anomaly in our marine records and none of the El Niņo mega-floods known from the continent date within the marine anomaly."  In addition, they report that "this precipitation anomaly also occurred in other high-resolution records throughout the ENSO domain," citing eleven other references in support of this statement.

What it means
Because of the facts that (1) heavy winter rainfalls along and off coastal Peru only occur during times of maximum El Niņo strength and (2) El Niņos are typically much more prevalent and stronger during cooler as opposed to warmer periods [see El Niņo (Relationship to Global Warming) in our Subject Index], the implied lack of strong El Niņos during the period of time from A.D. 800-1250 suggests that this period was truly a Medieval Warm Period.

Interestingly, the significance of this observation was not lost on Rein et al. In the introduction to their paper, they note that "discrepancies exist between the Mann curve and alternative time series for the Medieval period."  Most notably, to use their words, "the global Mann curve has no temperature optimum, whereas the Esper et al. (2002) reconstruction shows northern hemisphere temperatures almost as high as those of the 20th century" during the Medieval period.  And in the final sentence of their paper they suggest that "the occurrence of a Medieval climatic anomaly (A.D. 800-1250) with persistently weak El Niņos [our italics] may therefore assist the interpretation of some of the regional discrepancies in thermal reconstructions of Medieval times," which is a polite way of suggesting that the Mann et al. (1998, 1999) hockeystick temperature history is deficient in not depicting the presence of a true Medieval Warm Period.

References
Esper, J., Cook, E.R. and Schweingruber, F.H.  2002.  Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability.  Science 295: 2250-2253.

Magillian, F.J. and Goldstein, P.S.  2001.  El Niņo floods and culture change: A late Holocene flood history for the Rio Moquegua, southern Peru.  Geology 29: 431-434.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1998.  Global-scale temperature patterns and climate forcing over the past six centuries.  Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1999.  Northern Hemisphere temperatures duing the past millennium: Inferences, uncertainties, and limitations.  Geophysical Research Letters 26: 759-762.

Philander, S.G.H.  1990.  El Niņo, La Niņa, and the Southern Oscillation.  Academic Press, San Diego, California, USA.

Wells, L.E.  1990.  Holocene history of the El Niņo phenomenon as recorded in flood sediments of northern coastal Peru.  Geology 18: 1134-1137.


Reviewed 10 November 2004