Holocene climate dynamics in the central Mediterranean inferred from pollen data

The Mediterranean climate is characterised by strong seasonality, which is critical for the ecosystems and societies in the region and makes them susceptible to climate change. The timing of when the Mediterranean climate developed over the past few thousand years remains a complex and unresolved question. Most studies document a part of the Mediterranean area or are based on a single (and frequently different) climate reconstruction method, which can lead to non-negligible biases when considering climate changes on a Mediterranean scale. Several climate summaries based on pollen data have recently been produced on a European scale. However, few of them have focused exclusively on the Mediterranean area, except for two recent syntheses documenting the eastern and western parts of the Mediterranean basin. We aimed to document the climate changes in the central Mediterranean during the Holocene, including trends and different patterns. A robust methodology has been applied to 38 pollen records spreading across the south of France and Italy. Four climate reconstruction methods based on different mathematical and ecological concepts have been tested (MAT, WA-PLS, BRT and RF), and the selection of the best modern calibration dataset has also been investigated to produce the most reliable results. Particular attention has been paid to the seasonal nature of climatic parameters (winter and summer temperatures and precipitation). A model-data comparison has been made using the transient model simulation TraCE-21ka in an attempt to gain a better understanding of the climate mechanisms and their forcing. Our palaeoclimate reconstruction shows that during the mid Holocene, summer temperatures were slightly colder than modern-day conditions in the southern part of the central Mediterranean region, which is not completely in accordance with the summer temperature reconstructions of the Iberian Peninsula and eastern Mediterranean for the mid Holocene. In northern parts of the central Mediterranean region, and particularly in high elevation (> 1000 m), a Holocene thermal maximum is present, contrasting with the cold summer temperature anomalies previously reconstructed with pollen data for the Mediterranean region. Holocene summer conditions were characterised by specific spatio-temporal patterns, i.e., a west-east differentiation in southern France and a north-south one in Italy, for both temperature and precipitation. Holocene winter conditions showed a more homogeneous spatio-temporal pattern, i.e., general humidification and warming throughout the Holocene for Italy and southern France, which is coherent with the winter temperature reconstructions of the Iberian Peninsula and eastern Mediterranean. A data-model comparison shows a mostly coherent signal in winter but an incoherent one in summer. Those discrepancies between model simulations and pollen-based reconstructions suggest that during the Holocene, the northern Mediterranean climate was already subject to a marked spatio-temporal variability, particularly in summer, that cannot only be explained by changes in orbital configuration and atmospheric greenhouse gas evolution. Finally, our result highlighted the onset of the "Mediterraneanization" of the central Mediterranean region, characterised by wet winters and dry summers, after 8000 years Before Present (BP). The "Mediterraneanization" process seems to have had a greater impact on the southern regions than on the northern regions.