Frequency and intensity of palaeofloods at the interface of Atlantic and Mediterranean climate domains

The long-term response of the flood activity to both Atlantic and Mediterranean climatic influences was explored by studying a lake sequence (Lake Foreant) of the Western European Alps. High-resolution sedimentological and geochemical analysis revealed 171 event layers, 168 of which result from past flood events over the last millennium. The layer thickness was used as a proxy of intensity of past floods. Because the Foreant palaeoflood record is in agreement with the documented variability of historical floods resulting from local and mesoscale, summer-to-autumn convective events, it is assumed to highlight changes in flood frequency and intensity related to such events typical of both Atlantic (local events) and Mediterranean (mesoscale events) climatic influences. Comparing the Foreant record with other Atlantic-influenced and Mediterranean-influenced regional flood records highlights a common feature in all flood patterns that is a higher flood frequency during the cold period of the Little Ice Age (LIA, AD 1300-1900). In contrast, high-intensity flood events are apparent during both the cold LIA and the warm Medieval Climate Anomaly (MCA, AD 9501250). However, there is a tendency towards higher frequencies of high-intensity flood events during the warm MCA. The MCA extremes could mean that under the global warming scenario, we might see an increase in intensity (not in frequency). However, the flood frequency and intensity in the course of the 20th century warming trend did not change significantly. Uncertainties in future evolution of flood intensity lie in the interpretation of the lack of 20th century extremes (transition or stable?) and the different climate forcing factors between the two periods (greenhouse gases vs. solar and/or volcanic eruptions)