Solar and volcanic fingerprints in tree-ring chronologies over the past 2000 years Petra Breitenmoser a, b, ⁎, Jürg Beer c , Stefan Brönnimann a, b , David Frank b, d , Friedhelm Steinhilber c, b , Heinz Wanner b a Institute of Geography, Climatology and Meteorology, University of Bern, Hallerstrasse 12, 3012 Bern, Switzerland b Oeschger Centre for Climate Change Research (OCCR), University of Bern, Zähringerstrasse 25, 3012 Bern, Switzerland c Swiss Federal Institute of Aquatic Science and Technology (EAWAG), Überlandstrasse 133, 8600 Dübendorf, Switzerland d Swiss Federal Research Institute WSL, Zürcherstrasse 111, 8903 Birmensdorf, Switzerland abstract article info Article history: Received 16 March 2011 Received in revised form 21 October 2011 Accepted 23 October 2011 Available online 29 October 2011 Keywords: TSI Climate variability Tree-ring proxies DeVries solar cycle Volcanic activity Past two millennia The Sun is the main driver of Earth's climate, yet the Sun's role in forcing decadal-to-centennial climate var- iations has remained controversial, especially in the context of understanding contributions of natural cli- mate forcings to continuing global warming. To properly address long-term fingerprints of solar forcing on climate, long-term, very high-resolution, globally distributed climate proxy records are necessary. In this study we compile and evaluate a near global collection of annually-resolved tree-ring-based climate proxies spanning the past two millennia. We statistically assess these records in both the time and frequency do- mains for solar forcing (i.e. Total Solar Irradiance; TSI) and climate variability with emphasis on centennial time scales. Analyses in the frequency domain indicate significant periodicities in the 208-year frequency band, corresponding to the DeVries cycle of solar activity. Additionally, results from Superposed Epoch Anal- ysis (SEA) point toward a possible solar contribution in the temperature and precipitation series. However, solar-climate associations remain weak, with for example no clear linkage distinguishable in the southwestern United States drought records at centennial time scales. Other forcing factors, namely volcanic activity, appear to mask the solar signal in space and time. To investigate this hypothesis, we attempted to extract volcanic sig- nals from the temperature proxies using a statistical modelling approach. Wavelet analysis of the volcanic contribution reveals significant periodicities near the DeVries frequency during the Little Ice Age (LIA). This remarkable and coincidental superposition of the signals makes it very difficult to separate volcanic and solar forcing during the LIA. Nevertheless, the “volcano free” temperature records show significant periodicities near the DeVries periodicity during the entire past 1500 years, further pointing to solar mechanisms and emphasising the need for solar related studies in the absence of strong multi-decadal volcanic forcing. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The Sun's role in climate variability has been a much discussed topic (e.g. Crowley, 2000; Rind, 2002; Shindell et al., 2003; Ammann et al., 2007; Gray et al., 2010; Haigh et al., 2010; Lean, 2010). The recent low and long-lasting minimum of solar cycle 23 – superimposed on generally high background solar activity during the past ~60 years (Abreu et al., 2008) – has put a renewed focus on the relationship between solar variability and its influence on climate on decadal to mil- lennial time scales during the past, present, and future. While variations in Total Solar Irradiance (TSI) can only be measured quantitatively since the satellite era, changes in solar activity are captured in a number of proxies and measures including cosmic ray neutron monitor counts, measurements of geomagnetic disturbance (aa-index), the incidence of aurorae, observations of sunspots, or con- centrations of 10 Be in ice cores and 14 C in tree-rings, respectively (Gray et al., 2010). However, only reconstructions based on the cosmo- genic radionuclides allow the study of the evolution of solar magnetic activity beyond multi-centennial time-scales in a quantitative manner (e.g. Stuiver and Quay, 1980; Muscheler et al., 2004; Vonmoos et al., 2006). For instance, Solanki et al. (2004) and Usoskin et al. (2006) reconstructed the decadal averaged sunspot number for the past approximately 10,000 years from measurements of 14 C. A different approach, using the 10 Be record to reconstruct Total Solar Irradiance (TSI) during the past 9300 years, was applied by Steinhilber et al. (2009). A TSI reconstruction using 14 C has recently been published by Vieira et al. (2011), generally confirming the 10 Be record. Although only the 11- and 22-year solar periodicities (the so- called Schwabe and Hale cycle, respectively) can be observed in instrumental–meteorological records, other solar cycles are known, e.g. at ~90 years (Gleissberg), ~208 years (DeVries cycle, a.k.a. Suess cycle), ~1000 years (Eddy cycle, Abreu et al., 2010), and ~2300 years (Hallstatt cycle) (e.g. Stuiver and Braziunas, 1989; Damon and Sonett, Palaeogeography, Palaeoclimatology, Palaeoecology 313-314 (2012) 127–139 ⁎ Corresponding author at: Institute of Geography, Climatology and Meteorology, University of Bern, Hallerstrasse 12, CH-3012 Bern, Switzerland. Tel.: +41 31 631 85 42; fax: +41 31 631 85 11. E-mail address: petra.breitenmoser@giub.unibe.ch (P. Breitenmoser). 0031-0182/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2011.10.014 Contents lists available at SciVerse ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo