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. At the junction of greenhouse and icehouse climate states, theEocene–Oligocene Transition (EOT) is a key moment in Cenozoic climatehistory. While it is associated with severe extinctions and biodiversityturnovers on land, the role of terrestrial climate evolution remains poorlyresolved, especially the associated changes in seasonality. Somepaleobotanical and geochemical continental records in parts of the NorthernHemisphere suggest the EOT is associated with a marked cooling in winter,leading to the development of more pronounced seasons (i.e., an increase in themean annual range of temperature, MATR). However, the MATR increase has beenbarely studied by climate models and large uncertainties remain on itsorigin, geographical extent and impact. In order to better understand anddescribe temperature seasonality changes between the middle Eocene and theearly Oligocene, we use the Earth system model IPSL-CM5A2 and a set ofsimulations reconstructing the EOT through three major climate forcings:pCO2 decrease (1120, 840 and 560 ppm), the Antarctic ice-sheet (AIS)formation and the associated sea-level decrease. Our simulations suggestthat pCO2 lowering alone is not sufficient to explain the seasonalityevolution described by the data through the EOT but rather that thecombined effects of pCO2, AIS formation and increased continentalityprovide the best data–model agreement. pCO2 decrease induces a zonalpattern with alternating increasing and decreasing seasonality bandsparticularly strong in the northern high latitudes (up to 8 ∘CMATR increase) due to sea-ice and surface albedo feedback. Conversely, theonset of the AIS is responsible for a more constant surface albedo yearly,which leads to a strong decrease in seasonality in the southern midlatitudes tohigh latitudes (&gt;40∘ S). Finally, continental areasthat emerged due to the sea-level lowering cause the largest increase inseasonality and explain most of the global heterogeneity in MATR changes(ΔMATR) patterns. The ΔMATR patterns we reconstruct aregenerally consistent with the variability of the EOT biotic crisis intensityacross the Northern Hemisphere and provide insights on their underlyingmechanisms.

Evolution of continental temperature seasonality from the Eocene greenhouse to the Oligocene icehouse –a model–data comparison