Volume 9, Number 17: 26 April 2006
D'Arrigo et al. (2006) note that the Northern Hemispheric temperature reconstruction of Mann et al. (1999), which is the one preferred by most climate alarmists, "demonstrates minimal temperature amplitude (e.g., during the 'Medieval Warm Period' and 'Little Ice Age') while others (Briffa, 2000; Esper et al., 2002; Cook et al., 2004; Moberg et al., 2005) exhibit more pronounced variability." Consequently, in an effort to determine the reasons for this discrepancy, they assembled mostly tree-ring width (but some density) data from living and subfossil wood of coniferous tree species found at 66 high-elevation and latitudinal treeline North American and Eurasian sites, after which they analyzed these data via both Standard (STD) and Regional Curve Standardization (RCS) detrending techniques, the former of which (used by Mann et al.) does not do a good job of capturing the low-frequency variability that is required to accurately compare the temperatures of periods separated in time by many hundreds of years or more. In addition, D'Arrigo et al. report that "the North American data are much improved with new or extended millennial-length records, and updates of most of the data sets until at least the late 1990s." Also, they say they did not utilize the long bristlecone pine data sets from Colorado and California that Mann et al. employed, "as many appear to portray a mixed precipitation and temperature signal (in addition to a purported CO2 fertilization effect)," and that they did not use the Mackenzie Mountains, Boreal, Upperwright and Gotland data sets utilized by Esper et al. (2002) because they either "(1) did not demonstrate a significant temperature signal on the local to regional scale, (2) displayed significant correlations with precipitation, or (3) were located at lower latitudes than those compiled for the present analysis."
D'Arrigo et al.'s STD and RCS Northern Hemispheric (NH) temperature reconstructions spanned the period AD 713-1995; and, as may have been expected, they found that "the long-term trends of the STD reconstruction most closely match the Mann et al. (1999) and Jones et al. (1998) series, whereas the RCS reconstruction compares best with the Esper et al. (2002) and Cook et al. (2004) series." This observation, in their words, "validates the hypothesis (Esper et al., 2004) that one reason for the relative lack of long-term variability in the work of Mann et al. (1999) was their use of standard detrending procedures that removed low-frequency variation." On this point they thus conclude that "the RCS reconstruction is superior to the more traditional STD method with regards to the ability to retain low-frequency (centennial to multi-centennial) trends."
In comparing the temperatures of the Medieval Warm Period (MWP) with those of the Current Warm Period (CWP), based on the six longest (>1000 years) chronologies they analyzed, D'Arrigo et al. concluded that "the recent period does not look particularly warmer compared to the MWP." However, they note that the mean of the six series does depict a warmer CWP; but they describe this apparent relationship as "a bias/artifact in the full RCS reconstruction (and likely in many of the other reconstructions) where the MWP, because it is expressed at different times in the six long records, is 'averaged out' (i.e., flattened) compared to the recent period which shows a much more globally consistent signal."
Based on these findings, D'Arrigo et al. correctly concluded that "not enough proxy records yet exist for this time," i.e., for the MWP. Nevertheless, they had to work with what they had, and in doing so they found that "late twentieth century warming exceeds peak MWP conditions by 0.67°C when comparing decadal averages (960-969 (reconstruction) = -0.12°C versus 1991-2000 (instrumental) = 0.55°C [our italics])." This conclusion, of course, is based on an "apples and oranges" comparison; and the three researchers report that "peak twentieth century warmth for the period covered only by the proxy data (1937-1946, 0.17°C) exceeds peak MWP conditions by [only] 0.29°C," which is a significantly smaller number than that obtained by comparing the reconstructed and instrumental results.
A further confounding fact, according to D'Arrigo et al., is the "apparent decrease in recent temperature sensitivity for many northern sites ... with divergence from instrumental temperatures after ~1986." So great is this divergence, in fact, the late 1990s instrumental temperatures are essentially a full degree Centigrade higher than those of the proxy reconstructions. Hence, the three researchers were forced to conclude that "until valid reasons for this phenomenon have been found, [we] can only question the ability of tree-ring data to robustly model earlier periods that could have been similarly warm (or warmer) than the present."
In concluding their paper, D'Arrigo et al. suggest that to ultimately resolve the issue of the relative warmth of the CWP compared to that of the MWP, "many long records from new NH locations and updating of existing records to the present are required." This is our position as well; and that is why we have embarked upon our Medieval Warm Period Project, where each week we post the findings of a new experimental study that either (1) quantitatively or (2) qualitatively compares the warmth of the two periods and/or (3) specifies the MWP's time of occurrence. Only by acquiring a much greater data base than what has been obtained to date will we be able to conclusively determine whether CWP temperatures are indeed unprecedented over the past millennium, or whether they are the same as - or cooler than - those of the MWP. And if recent temperatures are not unprecedented, as we believe to be the case in light of the analyses we have seen to date, it should be clear to all that whatever was the cause of the warmth of the MWP could well be the cause of the warmth of the CWP.
Sherwood, Keith and Craig Idso
References
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Cook, E., Esper, J. and D'Arrigo, R. 2004. Extra-tropical Northern Hemisphere land temperature variability over the past 1000 years. Quaternary Science Reviews 23: 2063-2074.
D'Arrigo, R., Wilson, R. and Jacoby, G. 2006. On the long-term context for late twentieth century warming. Journal of Geophysical Research 111: 10.1029/2005JD006352.
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