Increased seasonality through the Eocene to Oligocene transition in northern high latitudes

Seasonal adjustment At about 33.5 million years ago — at the Eocene-Oligocene transition — Earth's climate switched from 'greenhouse' to 'icehouse' conditions. Continental-scale ice sheets appeared in Antarctica at about this time, together with reductions in atmospheric CO 2 , but, particularly for high northern latitudes, relatively little is know about what was happening to the climate at that time. To find out more about conditions in the Northern Hemisphere, Eldrett et al . analysed terrestrial spore and pollen preserved in ocean sediments in the Norwegian–Greenland Sea. They find that the coldest month mean temperatures declined by about 5 °C prior to the Eocene–Oligocene transition and that seasonality increased. About 33.5 million years ago, at the Eocene–Oligocene transition, the Earth's climate switched from greenhouse to icehouse conditions. The analysis of terrestrial spore and pollen evidence deposited in ocean sediments in the Norwegian–Greenland Sea now reveals that cold-month mean temperatures declined by about 5 °C prior to the Eocene–Oligocene transition and that seasonality increased. A profound global climate shift took place at the Eocene–Oligocene transition (∼33.5 million years ago) when Cretaceous/early Palaeogene greenhouse conditions gave way to icehouse conditions 1 , 2 , 3 . During this interval, changes in the Earth’s orbit and a long-term drop in atmospheric carbon dioxide concentrations 4 , 5 , 6 resulted in both the growth of Antarctic ice sheets to approximately their modern size 2 , 3 and the appearance of Northern Hemisphere glacial ice 7 , 8 . However, palaeoclimatic studies of this interval are contradictory: although some analyses indicate no major climatic changes 9 , 10 , others imply cooler temperatures 11 , increased seasonality 12 , 13 and/or aridity 12 , 13 , 14 , 15 . Climatic conditions in high northern latitudes over this interval are particularly poorly known. Here we present northern high-latitude terrestrial climate estimates for the Eocene to Oligocene interval, based on bioclimatic analysis of terrestrially derived spore and pollen assemblages preserved in marine sediments from the Norwegian–Greenland Sea. Our data indicate a cooling of ∼5 °C in cold-month (winter) mean temperatures to 0–2 °C, and a concomitant increased seasonality before the Oi-1 glaciation event. These data indicate that a cooling component is indeed incorporated in the δ 18 O isotope shift across the Eocene–Oligocene transition. Howev