Setting the tree-ring record straight
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Setting the tree‑ring record straight Josef Ludescher1 · Armin Bunde2 · Ulf Büntgen3,4,5,6 · Hans Joachim Schellnhuber1,7 Received: 5 May 2020 / Accepted: 18 August 2020 © The Author(s) 2020
Abstract Tree-ring chronologies are the main source for annually resolved and absolutely dated temperature reconstructions of the last millennia and thus for studying the intriguing problem of climate impacts. Here we focus on central Europe and compare the tree-ring based temperature reconstruction with reconstructions from harvest dates, long meteorological measurements, and historical model data. We find that all data are long-term persistent, but in the tree-ring based reconstruction the strength of the persistence quantified by the Hurst exponent is remarkably larger ( h ≅ 1.02 ) than in the other data ( h = 0.52–0.69), indicating an unrealistic exaggeration of the historical temperature variations.We show how to correct the tree-ring based reconstruction by a mathematical transformation that adjusts the persistence and leads to reduced amplitudes of the warm and cold periods. The new transformed record agrees well with both the observational data and the harvest dates-based reconstructions and allows more realistic studies of climate impacts. It confirms that the present warming is unprecedented. Keywords Tree-ring · Temperature reconstructions · Harvest dates · Long-term persistence · Hurst exponent
1 Introduction One of the important questions in global change research is which impact the increase of the recent warming will have on societies and human beings (Stocker et al. 2013). One possibility to study this question is to look at historical developments in relation to annually resolved and absolutely dated temperature reconstructions. Presently, the main source for these kinds of data are treering chronologies that in some cases cover more than two * Josef Ludescher josef.ludescher@pik‑potsdam.de 1
Potsdam Institute for Climate Impact Research, 14473 Potsdam, Germany
2
Institute for Theoretical Physics, Justus Liebig University Giessen, 35392 Giessen, Germany
3
Department of Geography, University of Cambridge, Cambridge CB2 3EN, UK
4
Swiss Federal Institute for Forest, Snow and Landscape Research, 8903 Birmensdorf, Switzerland
5
Global Change Research Centre (CzechGlobe), 603 00 Brno, Czech Republic
6
Department of Geography, Faculty of Science, Masaryk University, 603 00 Brno, Czech Republic
7
Tsinghua University, Beijing 100084, China
millennia (Esper et al. 2016). However, the reliability of their high-frequency variability (Matalas 1962; Cook et al. 1999) as well as, their low-frequency variability (von Storch et al. 2004; Moberg 2012; Franke et al. 2013; Bunde et al. 2013) is debated and it is not yet clear to which extent treering chronologies reflect historical reality. The purpose of this article is to assess and improve treering based reconstructions of temperatures. We focus on central Europe, where the temperatures of the past can be obtained not only from tree-rings (Büntgen et a
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