One of the most worrisome aspects of anthropogenic climate change is its potential to enhance the frequency and severity of extreme weather events and seasonality (van der Wiel et al., 2021). More accurate reconstructions of short-term climate variability in past warm climates help improve our projections of this type of variability in future climate (IPCC, 2013). Here, we apply our recently developed clumped isotope methodology for absolute seasonal sea surface temperature and salinity reconstructions (de Winter et al., 2021a; b) on fossil mollusk shells from the Pliocene Warm Period (3.0 – 3.3 Ma) of northwestern Europe, an important analogue for equilibrium climate under present-day radiative forcing (pCO2 ≈ 400 ppmV; Haywood et al., 2016). Isotope records from well-preserved shells of four different bivalve species (Arctica islandica, Glycymeris radiolyrata, Angulus benedeni and Ostrea edulis) reveal warm sea surface temperatures and high seasonal variability during the key mid-Pliocene PRISM interval, allowing detailed comparison with long-term geological climate reconstructions and an ensemble of model simulations (Haywood et al., 2016). Moreover, our results shed light on sub-annual variability in water chemistry in the shallow European epicontinental seas during this crucial period. These new findings highlight the effect of a warming climate on shallow marine ecosystems and shed light on the seasonal response to global warming. Haywood, A. M. et al. Integrating geological archives and climate models for the mid-Pliocene warm period. Nat Commun 7, 10646 (2016). IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, 1535 pp. (Cambridge Univ. Press, Cambridge, UK, and New York, 2013). van der Wiel, K. & Bintanja, R. Contribution of climatic changes in mean and variability to monthly temperature and precipitation extremes. Commun Earth Environ 2, 1–11 (2021). de Winter, N. J. et al. Optimizing sampling strategies in high-resolution paleoclimate records. Climate of the Past 17, 1315–1340 (2021a). de Winter, N. J. et al. Absolute seasonal temperature estimates from clumped isotopes in bivalve shells suggest warm and variable greenhouse climate. Commun Earth Environ 2, 1–8 (2021b).