Environmental conditions during the past 24 million years are thought to have been favourable for enhanced rates of atmospheric carbon dioxide drawdown by silicate chemical weathering. Proxy records indicate, however, that the Earth's atmospheric carbon dioxide concentrations did not fall below about 200

About 34 million years ago, Earth's climate shifted from a relatively ice-free world to one with glacial conditions on Antarctica characterized by substantial ice sheets. How Earth's temperature changed during this climate transition remains poorly understood, and evidence for Northern Hemisphere polar ice is controversial.

Sudden global warming 55 million years ago provides evidence for high climate sensitivity to atmospheric CO2, but the source of the carbon remains enigmatic.