Syntheses of pollen-based temperature reconstructions with respect to seasonal and spatiotemporal change in Europe

Abstract

The Holocene climate change plays a significant role in understanding the climate-human-environment interactions and predicting the future climate change. The Holocene temperature conundrum is one of the most debated topics, induced by the inconsistent results from reconstructed proxy records and climate simulation. The possible reason for this conundrum regarding proxy-based records is the potential bias of reconstructed temperatures towards summer conditions. Therefore, reconstructions of seasonal temperatures in different regions are crucial for solving the Holocene temperature conundrum. Pollen assemblages provide by far the most widely available proxies to reconstruct the Holocene climate change. Yet a synthesized record of the Holocene temperature reconstruction in Europe based on a sound integrated pollen compilation is still lacking. Here, we reconstruct the annual and seasonal temperature changes in Europe during the Holocene using different methods based on a large amount of fossil pollen records and analyze them using the whole and latitudinal synthesized subsets to investigate the spatio-temporal characteristics and to identify the driving mechanism of temperature changes. The annual and seasonal temperature reconstructions over the course of the Holocene in Europe show a general trend with a cold early Holocene towards a megathermal from early to middle Holocene followed by a slight cooling towards the present. A pervasive cold event can be observed at around 8 ka BP. The megathermal of the annual and summer temperature occurred earlier in the southern area (south of 50°N) than in the northern area (north of 50°N). The higher northern latitudes show more pronounced temperature variability. The driving mechanism of temperature change in Europe is also investigated by a comparison with the results of the TraCE-21ka simulation under full and single forcings during the Holocene, suggesting that European temperature is mainly controlled by orbital forcing, but is also influenced by other forcings such as ice sheet melting and atmospheric circulations. The consistency of the overall annual and seasonal temperature trends in the pollen-based reconstruction and the model simulation corroborates the reliability of our reconstruction results. This study provides synthesized results of Holocene seasonal temperature change at a pan-European scale for further understanding of the Holocene temperature conundrum and explores more about the latitudinal and seasonal temperature differences in Europe compared to the previous global and local reconstructions.