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A Two-Thousand-Year Temperature History of the Extra-Tropical Northern Hemisphere
Ljungqvist, F.C. 2010. A new reconstruction of temperature variability in the extra-tropical Northern Hemisphere during the last two millennia. Geografiska Annaler Series A 92: 339-351.

What was done
Fredrik Ljungqvist of Stockholm University's Department of History developed a 2000-year temperature history of the extra-tropical portion of the Northern Hemisphere (i.e., that part covering the latitudinal range 30-90°N) based on 30 temperature-sensitive proxy records with annual to multi-decadal resolution, including two historical documentary records, three marine sediment records, five lake sediment records, three speleothem δ18O records, two ice-core δ18O records, four varved thickness sediment records, five tree-ring width records, five tree-ring maximum latewood density records, and one δ13C tree-ring record, but not employing tree-ring width records from arid and semi-arid regions, since they may have been affected by drought stress, and they may not show a linear response to warming if higher summer temperatures also reduce the availability of water, as suggested by the work of D'Arrigo et al. (2006) and Loehle (2009).

What was learned
The results of the Swedish scientist's stellar efforts are depicted in the following figure.

Figure 1. Reconstructed extra-tropical (30-90°N) mean decadal temperature variations relative to the 1961-1990 mean of the variance-adjusted 30-90°N CRUTEM3+HadSST2 instrumental temperature data of Brohan et al.(2006) and Rayner et al. (2006). Adapted from Ljungqvist (2010).

In discussing this temperature history, Ljungqvist states that it depicts "a Roman Warm Period c. AD 1-300, a Dark Age Cold Period c. AD 300-800, a Medieval Warm Period c. AD 800-1300 and a Little Ice Age c. AD 1300-1900, followed by the twentieth-century warming." These alternating warm/cold periods, in his words, "probably represent the much discussed quasi-cyclical c. 1470 500-year Bond Cycles (Bond and Lotti, 1995; O'Brien et al., 1995; Bond et al., 1997, 2001; Oppo, 1997)," which "affected both Scandinavia and northwest North America synchronically (Denton and Karlen, 1973)" and have "subsequently also been observed in China (Hong et al., 2009a,b), the mid-latitude North Pacific (Isono et al., 2009) and in North America (Viau et al., 2006), and have been shown to very likely have affected the whole Northern Hemisphere during the Holocene (Butikofer, 2007; Wanner et al., 2008; Wanner and Butikofer, 2008), or even been global (Mayewski et al., 2004)."

Ljungqvist also notes that "decadal mean temperatures in the extra-tropical Northern Hemisphere seem to have equaled or exceeded the AD 1961-1990 mean temperature level during much of the Roman Warm Period and the Medieval Warm Period," and he says that "the second century, during the Roman Warm Period, is the warmest century during the last two millennia," while adding that "the highest average temperatures in the reconstruction are encountered in the mid to late tenth century," which was during the Medieval Warm Period. He warns, however, that the temperature of the last two decades "is possibly higher than during any previous time in the past two millennia," but adds that "this is only seen in the instrumental temperature data and not in the multi-proxy reconstruction itself," which is akin to saying that this possibility only presents itself if one applies Michael Mann's "Nature trick" of comparing "apples and oranges," which is clearly not valid.

What it means
This new study of Ljungqvist is especially important in that it utilizes, in his words, "a larger number of proxy records than most previous reconstructions," and that it "substantiates an already established history of long-term temperature variability." All of these facts, taken together, clearly demonstrate that there is nothing unusual, nothing unnatural or nothing unprecedented about the planet's current level of warmth, seeing it was just as warm as, or even warmer than, it has been recently during both the Roman and Medieval Warm Periods, when the atmosphere's CO2 concentration was more than 100 ppm less than it is today. And this latter observation, together with the realization that earth's climate naturally transits back and forth between cooler and warmer conditions on a millennial timescale, demonstrates that there is absolutely no need to associate the planet's current level of warmth with its current higher atmospheric CO2 concentration, in clear contradiction of the worn-out climate-alarmist claim that the only way to explain earth's current warmth is to associate it with the greenhouse effect of CO2. That claim -- for which there is no supporting evidence, other than misplaced blind faith in climate models -- is totally bogus.

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Bond, G. and Lotti, R. 1995. Iceberg discharges into the North Atlantic on millennial time scales during the last glaciation. Science 267: 1005-1010.

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Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P., deMenocal, P., Priore, P., Cullen, H., Hajdas, I. and Bonani, G. 1997. A pervasive millennial-scale cycle in North Atlantic Holocene and Glacial climate. Science 278: 1257-1266.

Butikofer, J. 2007. Millennial Scale Climate Variability During the Last 6000 Years -- Tracking Down the Bond Cycles. Diploma thesis, University of Bern, Bern, Switzerland.

Denton, G.H. and Karlen, W. 1973. Holocene climatic variations -- their pattern and possible cause. Quaternary Research 3: 155-205.

Hong, Y.T., Hong, B., Lin, Q.H., Shibata, Y., Zhu, Y.X., Leng, X.T. and Wang, Y. 2009a. Synchronous climate anomalies in the western North Pacific and North Atlantic regions during the last 14,000 years. Quaternary Science Reviews 28: 840-849.

Hong, B., Liu, C., Lin, Q., Yasuyuki, S., Leng, X., Wang, Y., Zhu, Y. and Hong, Y. 2009b. Temperature evolution from the δ18O record of Hami peat, Northeast China, in the last 14,000 years. Science in China Series D: Earth Sciences 52: 952-964.

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Rayner, N.A., Brohan, P., Parker, D.E., Folland, C.K., Kennedy, J.J., Vanicek, M., Ansell, T. and Tett, S.F.B. 2006. Improved analyses of changes and uncertainties in marine temperature measured in situ since the mid-nineteenth century: the HadSST2 dataset. Journal of Climate 19: 446-469.

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Reviewed 15 December 2010