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Terrestrial cooling in Northern Europe during the Eocene–Oligocene transition
Michael T. Hrena,b,1, Nathan D. Sheldonc, Stephen T. Grimesd, Margaret E. Collinsone, Jerry J. Hookerf, Melanie Buglerd, and Kyger C. Lohmannc

...Our data show a decrease in growing-season surface water temperatures (10 °C) during the Eocene–Oligocene transition, corresponding to an average decrease in mean annual air temperature of 4–6 °C from the Late Eocene to Early Oligocene. The magnitude of cooling is similar to observed decreases in North Atlantic sea surface temperature over this interval and occurs during major glacial expansion. This suggests a close linkage between atmospheric carbon dioxide concentrations, Northern Hemisphere temperature, and expansion of the Antarctic ice sheets.