At approximately 14 Ma, the East Antarctic Ice Sheet expanded to almost its current extent, but the role of CO₂ in this major climate transition is not entirely known. We show that atmospheric CO₂ might have varied on 400 kyr cycles linked to the eccentricity of the Earth’s orbit. The resulting change in weathering and ocean carbon cycle affected atmospheric CO₂ in a way that CO₂ rose after Antarctica glaciated, helping to stabilize the climate system on its way to the “ice-house” world.