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Low-Frequency Signals in Long Tree-Ring Chronologies Reveal the Existence of Multi-Centennial-Scale Temperature Trends of the Past Millennium*
Volume 7, Number 5: 4 February 2004

If the Medieval Warm Period of a thousand years ago was truly warmer than, or merely as warm as, the Modern Warm Period in which we currently live, there is simply no basis for claiming that any of the warming that brought us out of the Little Ice Age was caused by the concomitant historical rise in the air's CO2 content (Idso, 1988).  This is the reason why proponents of legislation to reduce anthropogenic CO2 emissions are so incensed by the appearance of scientific papers challenging the IPCC-inspired conclusion that recent temperatures have been the warmest of the past millennium; for if it was as warm as it is today a thousand years ago, when the atmosphere's CO2 concentration was nearly 100 ppm less than it is currently, whatever made it that warm then could well be making it that warm now.  Consequently, appropriately-derived temperature reconstructions of the past thousand years are of critical importance in helping to resolve this issue.

The latest contribution to this important quest is the study of Esper et al. (2003), who processed several extremely long juniper ring width chronologies for the Alai Range of the western Tien Shan in Kirghizia in such a way as to preserve multi-centennial growth trends that are typically "lost during the processes of tree ring data standardization and chronology building (Cook and Kairiukstis, 1990; Fritts, 1976)."  In doing so, they used two techniques that maintain low frequency signals: long-term mean standardization (LTM) and regional curve standardization (RCS), as well as the more conventional spline standardization (SPL) technique that obscures (actually removes) such long-term trends.

Carried back in time a full thousand years, the SPL chronologies depict significant inter-decadal variations but no longer-term trends.  The LTM and RCS chronologies, on the other hand, show long-term decreasing trends until about AD 1600, broad minima from 1600 to 1800, and long-term increasing trends from about 1800.  As a result, in the words of Esper et al., "the main feature of the LTM and RCS Alai Range chronologies is a multi-centennial wave with high values towards both ends."

This grand result has essentially the same form as the Northern Hemisphere extratropic temperature history of Esper et al. (2002), which is vastly different from the notorious hockeystick temperature history of Mann et al. (1998, 1999) and Mann and Jones (2003), in that it depicts the existence of both the Little Ice Age and preceding Medieval Warm Period, which are nowhere to be found in the Mann and Company reconstruction.  And the new result - especially the LTM chronology, which has a much smaller variance than the RCS chronology - depicts several periods in the first half of the last millennium that were warmer than any part of the last century.  These periods include much of the latter half of the Medieval Warm Period and a good part of the first half of the 15th century, which has also been found to have been warmer than it is currently by McIntyre and McKitrick (2003) and by Loehle (2004), as described in our Editorial of 28 Jan 2004.

In commenting on their important findings, Esper et al. remark that "if the tree ring reconstruction had been developed using 'standard' detrending procedures only, it would have been limited to inter-decadal scale variation and would have missed some of the common low frequency signal."  We would also remark, with respect to the upward trend of their data since 1800, that a goodly portion of that trend may well have been due to the aerial fertilization effect of the concomitantly increasing atmospheric CO2 concentration, which is known to greatly stimulate the growth of trees [see Long-Term Studies (Woody Plants) in our Subject Index].  Properly accounting for this very real effect would make the warmer-than-present temperatures of the first half of the past millennium even warmer, relative to those of the past century, than what they appear to be in Esper et al.'s LTM and RCS reconstructions.  Last of all, we find it extremely gratifying that Donald Graybill appears as a coauthor of the Esper et al. paper, over a decade after his untimely death in 1993, the very same year he published another important paper on this specific subject (Graybill and Idso, 1993).

In conclusion, as ever more data continue to accumulate, and as more correct procedures are employed to analyze them, the world's true temperature history is becoming ever more clear ? and what's beginning to take shape will shortly spell the end of the sorry centerpiece of the IPCC's ill-conceived rush to judgment on both the nature and cause of post-Little Ice Age climate change.

Sherwood, Keith and Craig Idso

*This editorial is dedicated to the memory of John Daly, who fought long and hard to demonstrate the non-uniqueness of late 20th century temperatures.  He will yet be recognized by all as having been correct on this important point.

Cook, E.R. and Kairiukstis, L.A.  1990.  Methods of Dendrochronology: Applications in the Environmental Sciences.  Kluwer, Dordrecht, The Netherlands.

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

Esper, J., Shiyatov, S.G., Mazepa, V.S., Wilson, R.J.S., Graybill, D.A. and Funkhouser, G.  2003.  Temperature-sensitive Tien Shan tree ring chronologies show multi-centennial growth trends.  Climate Dynamics 21: 699-706.

Fritts, H.C.  1976.  Tree Rings and Climate.  Academic Press, London, UK.

Graybill, D.A. and Idso, S.B.  1993.  Detecting the aerial fertilization effect of atmospheric CO2 enrichment in tree-ring chronologies.  Global Biogeochemical Cycles 7: 81-95.

Idso, S.B.  1988.  Greenhouse warming or Little Ice Age demise: A critical problem for climatology.  Theoretical and Applied Climatology 39: 54-56.

Loehle, C.  2004.  Climate change: detection and attribution of trends from long-term geologic data.  Ecological Modelling 171: 433-450.

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.

McIntyre, S. and McKitrick, R.  2003.  Corrections to the Mann et al. (1998) proxy data base and Northern Hemispheric average temperature series.  Energy and Environment 14: 751-771.