Amplified seasonality in western Europe in a warmer world

Documenting the seasonal temperature cycle constitutes an essential step toward mitigating risks associated
with extreme weather events in a future warmer world. The mid- Piacenzian Warm Period (mPWP), 3.3 to 3.0 mil
lion years ago, featured global temperatures approximately 3°C above preindustrial 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 of fossil mollusk shells from
the North Sea are presented to test Pliocene Model Intercomparison Project 2 outcomes. Joint data and model
evidence reveals enhanced summer warming (+4.3° ± 1.0°C) compared to winter (+2.5° ± 1.5°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 heat waves and other extreme weather events in Europe’s future.