Epoch· The columns of time

Oligocene

33.9 Ma → 23 Ma

The epoch nobody can quite explain: Antarctica carries an ice sheet close to its modern size, and the rest of the world stays warm anyway.

What the evidence shows

Sea surface temperatures reconstructed across its whole span, from 33.9 to 23.0 million years, show warm climates much like those of the late Eocene continuing with only brief interruptions, while the Antarctic ice sheet waxes and wanes. Carbon dioxide sits between about 300 and 700 parts per million, low by Cenozoic standards, and the late Oligocene appears to warm even as that figure declines, especially at high latitudes: warmth and carbon dioxide move in opposite directions, and current climate models account for it only in part. On one reading, the West Antarctic ice sheet retreats for a reason that has nothing to do with the air, a marine transgression driven by the tectonic sinking of the continental margin; marine ice returns only when the ocean itself cools at the Oligocene-Miocene transition.

Bounds and conventions

Its own specialists call it an enigma in a paper title. The bounds are defined, but the climate between them is poorly documented compared with what lies on either side, sea surface temperatures vary from place to place, and the authors of the fullest record across the epoch state plainly that the dominant drivers remain unclear.

Sources

1. Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
2. O'Brien et al. (2020), PNAS · The enigma of Oligocene climate and global surface temperature evolution
3. Duncan et al. (2022), Nature Geoscience · Climatic and tectonic drivers of late Oligocene Antarctic ice volume
4. Hutchinson et al. (2021), Climate of the Past · The Eocene-Oligocene transition: a review of proxy data, models and comparisons