Water vapour, CO 2 and insolation over the last glacial-interglacial cycles

• Published in: Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences
• Format: Article in Journal/Newspaper
• Language: English
• Published: The Royal Society 1993
• Subjects: ice core; Ice Sheet; Sea ice; taiga
• Online Access: http://dx.doi.org/10.1098/rstb.1993.0110; https://royalsocietypublishing.org/doi/pdf/10.1098/rstb.1993.0110

A two-dimensional model which links the atmosphere, the mixed layer of the ocean, the sea ice, the continents, the ice sheets and their underlying 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 drive the simulated climate system and in particular the feedbacks related to surface albedo and water vapour. It was found that orbital variations alone can induce, in such a system, feedbacks sufficient to generate the low frequency p art of the climatic variations over the last 122 ka. These simulated variations at the astronomical timescale are broadly in agreement with reconstructions of ice-sheet volume and of sea level independently obtained from geological data. Imperfections in the stimulated climate were the insufficient southward extent of the ice sheets and the too small hemispheric cooling at the last glacial maximum . 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 transient simulation, 70% of the Northern Hemisphere ice volume is related to the astronomical forcing and the related changes in the albedo, the rem aining 30% being due to the CO 2 changes. Analysis of the processes involved shows that variations of ablation are more important for the ice-sheet response than are variations of snow precipitation. A key mechanism in the deglaciation after the last glacial maxim um appears to be the ‘ageing’ of snow which significantly decreases its albedo. The other factors which play an important role are ice-sheet altitude, insolation, taiga cover, ice-albedo feedback, ice-sheet configuration (‘continentality’ and ‘desert’ effect), isostatic rebound, CO 2 changes and tem perature-water vapour feedback. Numerical experiments have also been carried out with a one-dimensional radiative-convective model in order to ...