Against the background of progressive cooling, three permanent warm periods occurred within the Cenozoic, i. e. the Early Eocene, the Middle Miocene, and the Pliocene. During the Cenozoic, the Earth evolved from an ice-free, to an Antarctica glacial, and finally into today’s planet with polar ice caps. These past warm intervals fail as satisfactory geologic analogs for a future warming. However, they provide a valuable opportunity to understand the processes that operated during warm climate states, especially on the climate sensitivity of various CO2 concentrations, and to improve the prediction qualities of climate models.