Niels J. de Winter , Julia Tindall, Andrew L.A. Johnson, Barbara Goudsmit-Hazevoort, Nina Wichern, Pim Kaskes, Philippe Claeys , Fynn Huygen, Sonja van Leeuwen, Brett Metcalfe, Pepijn Bakker, Stijn Goolaerts, Frank Wesselingh, Martin Ziegler

Documenting the seasonal temperature cycle constitutes an essential step towards mitigating risks associated with extreme weather events in a future warmer world. The mid-Piacenzian Warm Period (mPWP), 3.3 – 3.0 million years ago, featured global temperatures approximately 3°C above pre-industrial levels. It represents an ideal period for directed paleoclimate reconstructions equivalent to model projections for 2100 under moderate Shared Socioeconomic Pathway SSP2-4.5. Here, seasonal clumped isotope analyses in fossil mollusc shells from the North Sea are presented to test Pliocene Model Intercomparison Project 2 outcomes. Joint data and model evidence reveals enhanced summer warming (+4.2 ± 2.6 °C) compared to winter (+2.2 ± 2.0°C) during the mPWP, equivalent to SSP2-4.5 outcomes for future climate. We show that Arctic Amplification of global warming weakens mid-latitude summer circulation while intensifying seasonal contrast in temperature and precipitation, leading to an increased risk of summer heatwaves and other extreme weather events in Europe’s future.