Long-term summer (AD751-2008) temperature fluctuation in the French Alps based on tree-ring data
Résumé
On the basis of a dense tree-ring width network (34 unpublished multi-centennial larch chronologies), this paper attempts to reconstruct, for the first time, the summer temperatures in the French Alps (44 degrees-45.30 degrees N, 6.30 degrees-7.45 degrees E) during the last millennium. The adaptative Regional Growth Curve standardization method is applied to preserve interannual to multi-centennial variations in this high-elevation proxy data set. The proxies are calibrated using the June to August mean temperatures from the last revised version of the HISTALP database spanning the period AD1760-2003 and adjusted to take into account the warm bias before 1850. About 45% of the temperature variance is reconstructed. Despite the use of the newly updated meteorological data set, the reconstruction still shows colder temperatures than early instrumental measurements between 1760 and 1840. The proxy record evidences a prolonged Medieval Warm Period persisting until 1500, with warm periods that resemble 20th century conditions but also cold phases before 1000 synchronous with Swiss glacier advances. The Little Ice Age is rather mild until 1660 if compared with other Alpine reconstructions. Thereafter, summers are 0.7 degrees C cooler than the 1961-1990 mean until 1920. The maximum temperature amplitude over the past 1250 years is estimated to be 3 degrees C between the warmest (810s, 1990s) and coldest (1810s) decades. Most of the 20th century is comparable with the Medieval Warm Period.