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More Problems for the Mann et al. Temperature Record
Volume 7, Number 50: 15 December 2004

In the introduction to their most interesting and enlightening paper, Rein et al. (2004) note what is beginning to be painfully obvious to most thinking people (and hopefully causing them consternation), i.e., the fact that "discrepancies exist between the Mann curve and alternative time series for the Medieval period."  Most notably, as they put it, "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, when the atmosphere's CO2 concentration was much lower than it is today and was clearly, therefore, not responsible for the higher temperatures of those times, which in turn suggests that the higher CO2 levels of today may not be responsible for the higher temperatures of the Modern Warm Period relative to those of the preceding Little Ice Age.

In introducing their unique approach to helping to resolve this discrepancy, Rein et al. further note that "interannual climate variability along and off coastal Peru is dominated by ENSO," and that "in the hyperarid coastal deserts, heavy winter rainfalls only occur during ... years of maximum El Niņo strength (Philander, 1990)."  Hence, because periods of global warmth typically produce fewer El Niņos than periods that are cooler [see El Niņo (Relationship to Global Warming) in our Subject Index], they set about to see what they could learn about the relative merits of the two contrasting temperature histories of the past thousand-plus years (especially with regard to the existence or non-existence of a truly warm Medieval Warm Period) by looking for signs of ENSO activity in the sediments of a sheltered basin situated on the edge of the Peruvian shelf about 80 km west of Lima, Peru.

What Rein et al. discovered was that "lithic concentrations were very low for about 450 years during the Medieval climatic anomaly (MCA) from A.D. 800 to 1250," which would indeed suggest the existence of significantly warmer temperatures during this period.  In addition, they found 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," which also suggests that the Medieval Warm Period was warmer than the periods that preceded and followed it.

Other studies provide independent evidence for the same conclusion.  Rein et al. note, for example, that "from an Ecuadorian lake record where moderate to strong El Niņo floods are recorded (Moy et al., 2002), a minimum of such events is reported during the upper Medieval period."  Also, they note that the oldest (A.D. 928-961) of the five "time windows" on central Pacific El Niņo activity that is provided by the corals that were investigated by Cobb et al. (2003) exhibits evidence for weaker El Niņos than all subsequent time windows extending to 1998.  In addition, they report that "extreme long-lasting droughts that peaked coincident with those in the Peru record around A.D. 1160, are reported from several archives in the western USA and Southern Patagonia (Stine, 1994)," and they say that near-contemporaneous dry periods "also occurred in the tropical Andes (Abbot et al., 1997; Binford et al., 1997), Oman (Fleitmann et al., 2003) and eastern Africa (De Putter et al., 1998; Verschuren et al., 2000)."  Last of all, they state that "hints that these droughts are not only coinciding events but related to El Niņo anomalies come from the high-resolution Moon Lake (North Dakota, USA) salinity record (Laird et al., 1996)."

The significance of these observations resides in the fact that much palaeoclimatic research has demonstrated that El Niņo activity almost always decreases, and decreases substantially, as the planet warms [see again El Niņo (Relationship to Global Warming) in our Subject Index]; and as a result of this fact, Rein et al. are able to confidently state in their concluding sentence that "the occurrence of a Medieval climatic anomaly (A.D. 800-1250) with persistently weak El Niņos may therefore assist the interpretation of some of the regional discrepancies in thermal reconstructions of Medieval times."  And that assistance, as should be abundantly clear, helps us to realize, as is also evident from a host of proxy temperature records (see Medieval Warm Period in our Subject Index), that temperatures were indeed significantly cooler both before and after the Medieval period of A.D 800-1250, as well as throughout the 20th century; for with respect to the recent past, the data of Cobb et al. (2003) testify to the fact that current El Niņo activity has not yet dropped to the level characteristic of the Medieval Warm Period.

In light of these several observations, we are left with no alternative (as no one else should be either) but to conclude that the Northern Hemispheric temperature reconstruction of Esper et al. (2002) is superior to that of Mann et al. (1998, 1999), as it also is superior to the global temperature reconstruction of Mann and Jones (2003), both with respect to its demonstrating the existence of a truly warm Medieval Warm Period and the likelihood that that period of significant warmth was warmer than the Modern Warm Period has been to date, all without any help from elevated atmospheric CO2 concentrations (actually, in the face of very reduced Medieval Warm Period CO2 concentrations), which suggests that the variable atmospheric CO2 concentration of the planet has been vastly overrated as an agent of climate change.

Sherwood, Keith and Craig Idso

Abbot, M.B., Binford, M.W., Brenner, M. and Kelts, K.R.  1997.  A 3500 C14 yr high-resolution record of water-level changes in Lake Titicaca, Bolivia/Peru.  Quaternary Research 47: 169-180.

Binford, M.A., Kolata, M., Brenner, M., Janusek, L., Seddon, M., Abbott, M. and Curtis, J.  1997.  Climate variation and the rise and fall of an Andean civilization.  Quaternary Research 47: 235-248.

Cobb, K.M., Charles, C.D., Cheng, H. and Edwards, R.L.  2003.  El Niņo/Southern Oscillation and tropical Pacific climate during the last millennium.  Nature 424: 271-276.

De Putter, T., Loutre, M.-F. and Wansard, G.  1998.  Decadal periodicities of Nile River historical discharge (A.D. 622-1470) and climatic implications.  Geophysical Research Letters 25: 3193-3196.

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.

Fleitmann, D., Burns, S.J., Mudelsee, M., Neff, U., Kramers, J., Mangini, A. and Matter, A.  2003.  Holocene forcing of the Indian monsoon recorded in a stalagmite from southern Oman.  Science 300: 1737-1739.

Laird, K.R., Fritz, S.C., Maasch, K.A. and Cumming, B.F.  1996.  Greater drought intensity and frequency before AD 1200 in the northern Great Plains, USA.  Nature 384: 552-554.

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 during the past millennium: Inferences, uncertainties, and limitations.  Geophysical Research Letters 26: 759-762.

Mann, M.E. and Jones, P.D.  2003.  Global surface temperatures over the past two millennia.  Geophysical Research Letters 30: 10.1029/2003GL017814.

Moy, C.M., Seltzer, G.O., Rodbell, D.T. and Anderson D.M.  2002.  Variability of El Niņo/Southern Oscillation activity at millennial timescales during the Holocene epoch.  Nature 420: 162-165.

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

Rein, B., Luckge, A. and Sirocko, F.  2004.  A major Holocene ENSO anomaly during the Medieval period.  Geophysical Research Letters 31: 10.1029/2004GL020161.

Stine, S.  1994.  Extreme and persistent drought in California and Patagonia during mediaeval time.  Nature 369: 546-549.

Verschuren, D., Laird, K.R. and Cumming, B.F.  2000.  Rainfall and drought in equatorial east Africa during the past 1,100 years.  Nature 403: 410-414.

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