Millennium-long summer temperature variations in the European Alps as reconstructed from tree rings

This paper presents a reconstruction of the summer temperatures over the Greater Alpine Region (44.05°-47.41° N, 6.43°-13° E) during the last millennium based on a network of 38 multi-centennial larch and stone pine chronologies. Tree ring series are standardized using an Adaptative Regional Growth Curve, which attempts to remove the age effect from the low frequency variations in the series. The proxies are calibrated using the June to August mean temperatures from the HISTALP high-elevation temperature time series spanning the 1818-2003. The method combines an analogue technique, which is able to extend the too short tree-ring series, an artificial neural network technique for an optimal non-linear calibration including a bootstrap technique for calculating error assessment on the reconstruction. About 50% of the temperature variance is reconstructed. Low-elevation instrumental data back to 1760 compared to their instrumental target data reveal divergence between (warmer) early instrumental measurements and (colder) proxy estimates. The proxy record indicates cool conditions, from the mid-11th century to the mid-12th century, related to the Oort solar minimum followed by a short Medieval Warm Period (1200-1420). The Little Ice Age (1420-1830) appears particularly cold between 1420 and 1820 with summers that are 0.8 °C cooler than the 1901-2000 period. The new record suggests that the persistency of the late 20th century warming trend is unprecedented. It also reveals significant similarities with other alpine reconstructions