A robust spatial reconstruction of April to September temperature in Europe: Comparisons between the medieval period and the recent warming with a focus on extreme values

摘要

This report presents a study of the variability in a recently published April to September gridded reconstruction temperatures in Europe and an analysis of their extremes. This reconstruction was based on tree rings, historical documents, pollen assemblages and ice cores. The methodology used, an original spectral analog method, preserves long-term variations and the variability of temperature series, which guarantees the pertinence of the analysis of the extremes over a period as long as the last millennium. The analysis of the extremes was performed with the peak-over-threshold (POT) method over two warm periods (A.D. 1000-1350 and 1880-2007) and a cold period (A.D. 1350-1950). We found that (1) according to the long-term variations shown in this reconstruction, the growing season temperature during the last decade has exceeded all of those observed during the Medieval period; (2) the return period of the maximum event of the Medieval period has been reduced by at least a factor of 5; and (3) all decades before A.D. 1350 were warm on average but relatively heterogeneous, while the last decade was homogeneously warmer. A new result of this study concerning Europe is that this anthropogenic change is characterized by spatial homogeneity, with similar changes in both average temperatures and in the distribution of extreme events, while natural climate forcings induce warm periods with heterogeneous spatial patterns and less frequent extreme events.