Epoch· The columns of time

Oligocene

33.9 Ma → 23 Ma

In the Oligocene, from 34 to 23 million years ago, Antarctica carried an ice sheet close to its present size while the rest of the world stayed warm, a pairing that climate models reproduce only in part.

Epoch in which Antarctica carries an ice sheet close to its present size while the rest of the world remains warm, a configuration that climate models reproduce only in part.

What the evidence shows

Sea surface temperatures, reconstructed across its whole duration, from 33.9 to 23.0 million years, indicate conditions very close to those of the late Eocene, interrupted only by brief episodes, while the Antarctic ice sheet grows and shrinks. Carbon dioxide stands between about 300 and 700 parts per million, a low value on the Cenozoic scale, and the late Oligocene appears to warm even as that value falls, particularly at high latitudes: temperature and carbon dioxide move in opposite directions, which current climate models reproduce only imperfectly. On one interpretation, the West Antarctic ice sheet retreats for a non-atmospheric reason, a marine transgression caused by tectonic subsidence of the continental margin, sea ice returning only with the cooling of the ocean itself, at the Oligocene-Miocene transition.

Bounds and conventions

This is the epoch its own specialists call an enigma in a paper title. Its limits are defined, but its climate is poorly documented relative to the epochs around it, its surface temperatures vary between sites, and the authors of the most complete record crossing it state explicitly that the dominant drivers of its climate remain undetermined.