Rapid Swings between Greenhouse and Icehouse Climate States near the Oligocene-Miocene Boundary

The Earth's Cenozoic climate is conventionally portrayed as either being in a greenhouse or an icehouse conditions. Greenhouse climates are characterized by warm temperatures, high CO2 concentrations, low continental ice volume and reduced meridional temperature gradients, whereas icehouse climates are the opposite. The transition between greenhouse and icehouse primarily is achieved through stepwise and unidirectional cooling, ice sheet growth and increases in the meridional temperature gradients. Various feedbacks in the climate system and the global carbon cycle as well as the ice sheet hysteresis effect seem to preclude substantial fluctuations in the meridional temperature gradients, atmospheric CO2 concentrations and the volume of the East Antarctic Ice Sheet (EAIS) on a high frequency (orbital timescales). For example, relative to the Holocene, the last glacial maximum (LGM) is …