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1. Introduction
Millennia-long temperature reconstructions on regional-to-global scales provide important information for evaluating the performance of climate model simulations and can potentially offer an opportunity to reduce uncertainties in future climate change projections (Braconnot et al. 2012; Coats et al. 2013; Fernández-Donado et al. 2013; Phipps et al. 2013; Schmidt et al. 2014). Recently, the PAGES 2k Consortium (2013) reconstructed continental-scale temperature variations over the last one to two millennia, which were used in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5) for assessing paleoclimate model simulations on a continental scale (Masson-Delmotte et al. 2013). The geographically heterogeneous availability of temperature proxy data presently poses severe limitations for addressing changes on regional scales, and further improvements of continental-scale reconstructions are thus essential. This is, in particular, the case for Asia, where Cook et al. (2013) used 226 tree-ring chronologies of different length to reconstruct summer [June–August (JJA)] temperature variability since AD 800 as part of the PAGES 2k Consortium (2013). This reconstruction represented temperature variations well [r > 0.5 with the CRU Time Series, version 3.10 (TS3.10), temperature data; University of East Anglia CRU (2013)] over the higher latitudes of Asia (north of 36°N), but unfortunately had little skill (r < 0.2 with the CRU TS3.10 temperature data) over most of the middle and low latitudes of Asia (south of ~36°N) and especially over the Tibetan Plateau (TP) [see Fig. 11 in Cook et al. (2013)]. One reason for this may be the limited number of temperature-sensitive tree-ring chronologies used in the reconstruction by Cook et al. (2013) from the TP. Another underlying reason could be the spatial heterogeneity of temperature variability over much of Asia, which poses a challenge to large-scale (e.g., continental-to-hemispheric scales) temperature reconstructions using tree-ring chronologies representing local-to-regional-scale temperature variability (Esper et al. 2002; D’Arrigo et al. 2006; Büntgen et al. 2010). An additional challenge for this particular region is the relatively short and often incomplete instrumental temperature records, which makes it difficult to produce robust calibration and validation statistics because of the limited degrees of freedom (You et al. 2008; Liu et al. 2009; Cook et al. 2013).
The TP, with an average elevation exceeding 4000 m above mean sea level, is the...