Large parts of Antarctica became suddenly and substantially icy about 34 million years ago. Oxygen-isotope records suggest that a simultaneous accumulation in ice cover happened in the Northern Hemisphere, which is now challenged by Zhonghui Liu of Yale University and his colleagues.
They calculate sea-surface temperatures for the period using a pair of chemical proxies (tetraether and unsaturated alkenone) found in sediment cores retrieved from 11 locations around the world. According to their calculations, high-latitude cooling averaged 5 °C.
After plugging their numbers into an ocean-circulation model, the team calculated that ocean cooling could explain the discrepancy in oxygen-isotope records: a Northern Hemisphere glaciation would not have been required.