WATER-VAPOR, CO2 AND INSOLATION OVER THE LAST GLACIAL INTERGLACIAL CYCLES

A two-dimensional model which links the atmosphere, the mixed layer of the ocean, the sea ice, the continents, the ice sheets and their unde rlying bedrock has been used to test the Milankovitch theory over the last two glacial-interglacial cycles. A series of sensitivity analyses have allowed us to understand better the internal mechanisms which dr ive the simulated climate system and in particular the feedbacks relat ed to surface albedo and water vapour. It was found that orbital varia tions alone can induce, in such a system, feedbacks sufficient to gene rate the low frequency part of the climatic variations over the last 1 22 ka. These simulated variations at the astronomical timescale are br oadly in agreement with reconstructions of ice-sheet volume and of sea level independently obtained from geological data. Imperfections in t he stimulated climate were the insufficient southward extent of the ic e sheets and the too small hemispheric cooling at the last glacial max imum. These deficiencies were partly remedied in a further experiment by using the time-dependent atmospheric CO2 concentration given by the Vostok ice core in addition to the astronomical forcing. In this tran sient simulation, 70% of the Northern Hemisphere ice volume is related to the astronomical forcing and the related changes in the albedo, th e remaining 30% being due to the CO2 changes. Analysis of the processe s involved shows that variations of ablation are more important for th e ice-sheet response than are variations of snow precipitation. A key mechanism in the deglaciation after the last glacial maximum appears t o be the 'ageing' of snow which significantly decreases its albedo. Th e other factors which play an important role are ice-sheet altitude, i nsolation, taiga cover, ice-albedo feedback, ice-sheet configuration ( 'continentality' and 'desert' effect), isostatic rebound, CO2 changes and temperature-water vapour feedback. Numerical experiments have also been carried out with a one-dimensional radiative-convective model in order to quantify the influence of the CO2 changes and of the water v apour feedback on the climate evolution of the Northern Hemisphere ove r the last 122 ka. Results of these experiments indicate that 67% of t he simulated cooling at the last glacial maximum can be attributed to the astronomical forcing and the subsequent surface albedo increase, t he remaining 33% being associated with the reduced CO2 concentration. Moreover, the water vapour feedback explains 40% of the simulated cool ing in all the experiments done. The transient response of the climate system to both the astronomical and CO2 forcing was also simulated by the LLN (Louvain-la-Neuve) 2.5-dimensional model over the two last gl acial-interglacial cycles. It is particularly significant that spectra l analysis of the simulated Northern Hemisphere global ice volume vari ations reproduces correctly the relative intensity of the peaks at the orbital frequencies. Except for variations with timescales shorter th an 5 ka, the simulated long-term variations of total ice volume are co mparable to that reconstructed from deep sea cores. For example, the m odel simulates glacial maxima of similar amplitudes at 134 ka BP and 1 5 ka BP, followed by abrupt deglaciations. The complete deglaciation o f the three main Northern Hemisphere ice sheets, which is simulated ar ound 122 ka BP, is in partial disagreement with reconstructions indica ting that the Greenland ice sheet survived during the Eemian interglac ial. The continental ice volume variations during the last 122 ka of t he 200 ka simulation are, however, not significantly affected by this shortcoming.