Clim. Past Discuss., 7, 3991-4035, 2011
© Author(s) 2011. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Climate of the Past (CP). Please refer to the corresponding final paper in CP.
The extra-tropical NH temperature in the last two millennia: reconstructions of low-frequency variability
B. Christiansen1 and F. C. Ljungqvist2
1Danish Meteorological Institute, Copenhagen, Denmark
2Department of History, Stockholm University, Stockholm, Sweden

Abstract. We present two new multi-proxy reconstructions of the extra-tropical Northern Hemisphere (30–90° N) mean temperature: a two-millennia long reconstruction reaching back to AD 1 based on 32 proxies and a 500-yr long reconstruction reaching back to AD 1500 based on 91 proxies. The proxies are of different types and of different resolutions (annual, annual-to-decadal, and decadal) but all have previously been shown to relate to local or regional temperature. We use a reconstruction method, LOC, that recently has been shown to confidently reproduce low-frequency variability. Confidence intervals are obtained by an ensemble pseudo-proxy method that both estimates the variance and the bias of the reconstructions. The two-millennia long reconstruction shows a well defined Medieval Warm Period with a peak warming ca. AD 950–1050 reaching 0.7 °C relative to the reference period AD 1880–1960. The 500-yr long reconstruction confirms previous results obtained with the LOC method applied to a smaller proxy compilation; in particular it shows the Little Ice Age cumulating in AD 1580–1720 with a temperature minimum of −1.1 °C below the reference period. The reconstructed local temperatures, the magnitude of which are subject to wide confidence intervals, show a rather geographically homogeneous LIA while more geographical inhomogeneities are found for the Medieval Warm Period. Reconstructions based on different number of proxies show only small differences suggesting that LOC reconstructs 50-yr smoothed extra-tropical NH mean temperatures well and that low-frequency noise in the proxies is a relatively small problem.

Citation: Christiansen, B. and Ljungqvist, F. C.: The extra-tropical NH temperature in the last two millennia: reconstructions of low-frequency variability, Clim. Past Discuss., 7, 3991-4035, doi:10.5194/cpd-7-3991-2011, 2011.
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