Last-millennium summer-temperature variations in Brianconnais (French Alps) based on a composite tree-ring larch chronology
Résumé
We present a tree-ring-based reconstruction of last millenium summer temperature from Briançonnais in the northern French Alps. The sample material is derived from multi-centennial living trees and also includes larch (Larix decidua Mill.) series from historical construction timbers. The Regional Curve Standardization method was applied to preserve interannual to multicentennial variations in this high-elevation dataset. The proxies are inferred using a linear model and calibrated using the June to August mean temperatures from the last revised version of the HISTALP database spanning the 1760-2003 and adjusted to take into account warm bias before 1850. The transfer function has cross-validated coefficients of correlation (r) of 0.48 (0.42), reductions of error (RE) of 0.21 (0.27) and coefficients of efficiency of 0.20 (0.16). The concordance of the Briançonnais temperature reconstruction with other Alpine records suggests that the transfer function has successfully reconstructed past summer temperature during the last two millennia. Evidence was found of a warm episode between and 900 and 1200AD, and a cold phase between 1500 and 1850AD. These events were possibly correlated to the so-called 'Mediaeval Warm Period' (MWP) and 'Little Ice Age' (LIA). model reconstructed a summer temperature decrease more than 2°C for the LIA compared with the temperature prevailing during the MWP.
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