A European monsoon-like climate in a warmhouse world.

The middle Eocene warmhouse period (45 million years ago) featured atmospheric carbon dioxide concentrations equivalent to those projected under high future emission scenarios. Seasonal- to weather timescale climate reconstructions from this period can provide critical insight into the impact of Anthropogenic warming on intra-annual variability in temperature and precipitation. Here, we combine daily-scale reconstructions of the evolution of temperature and the water cycle in western Europe based on stable oxygen and clumped isotope analyses on the fastest-growing gastropod known in the fossil record: Campanile giganteum. Our dataset shows that the middle Eocene of western Europe featured monsoon-like conditions, with seawater temperatures of ~24 °C during mild and wet winters, >30 °C during hot and dry spring and autumn seasons, and ~28 °C during warm and comparatively wet summers. Coupled climate model simulations using the Community Earth System Model indicate these seasonal variations in temperature and precipitation were driven by shifting atmospheric and oceanic circulation regimes over Western Europe, with winds from different directions bringing distinct waters to the region and minimal wind during spring reducing cooling through diminished latent heat flux. Our results highlight that Europe may experience wetter summers with more frequent extreme rainfall events under future high emissions scenarios.