FEEDBACKS IN THE RESPONSE OF THE LLN CLIMATE MODEL TO THE ASTRONOMICAL FORCING

The LLN 2-D climate model has been used to reconstruct the long-term climatic variations over the Quaternary Ice Age. Sensitivity analyses to the astronomically-driven insolation changes and to the CO₂ atmospheric concentration have been performed. Under such conditions, the model simulates the intensification of glaciation around 2.75 Myr BP, the late Pliocene-early Pleistocene 41-kyr cycle, the emergence of the 100-kyr cycle around 900 kyr BP, and the glacial-interglacial cycles of the last 600 kyr. The insolation profile at stages 11 and 1 and modeling results tend to show that stage 11 might be a better analogue for our future climate than the Eem. Although the insolation changes alone act as a pacemaker for the glacial-interglacial cycles, CO₂ changes help to better reproduce past climatic changes and, in particular, the air temperature and the southern extend of the ice sheets. Using the calculated insolation and a few scenarios for CO₂, the climate of the next 130 kyr has been simulated. It shows that our interglacial will most probably last particularly long (50 kyr). This conclusion is reinforced if we take into account the possible intensification of the greenhouse effect which might result from man's activities over the next centuries.

The most important feedback mechanisms in the LLN model are related to albedo and temperature, water vapor and temperature, snow albedo and land cover, sea level and ice volume, ice sheets and isostatic rebound, ice sheets and altitude and continentality, and snow ageing. Although some are more important than others, no single one can alone reproduce the complexity of the glacial-interglacial cycles. Finally, it has been shown that the low frequency changes of climate at the astronomical time scale are a pre-requisite framework for producing the high frequency changes through nonlinearities related to land cover, sea ice and ocean dynamics.