Palaeoclimate sensitivity to CO2 and insolation

The current Quaternary Ice Age is characterized by multiple switches of the global climate between glacials and interglacials. The Louvain-la-Neuve two-dimensional (altitude-latitude) climate model (LLN 2-D model) has been used to simulate the long-term variations of the Northern Hemisphere climate. The model is forced by both the CO2 and the insolation for each day and latitude. The period analyzed here is from 200 ka BP to 200 ka AP. The model provides a total continental ice volume, which compares relatively well with the SPECMAP oxygen-isotope record, the size and extent of the individual Northern Hemisphere ice sheets being in fair agreement with their geological reconstructions. Seven simulations were done to quantify the relative contribution of the insolation and CO2 forcings and of the albedo and water-vapor temperature feedbacks to the last glacial maximum (LGM) cooling. This LGM cooling of 4.5 degrees C would be due to CO2 alone for 20.8%, to insolation and the albedo temperature feedback for 40%, and to the water vapor feedback for 38.6%. If we consider the low CO2 concentration as the only forcing, but allow water vapor to change, 33% of the cooling are explained