Records of global ice volume for the past 700 kyr, based on oxygen isotopic data from deep-sea cores and reflecting mainly the changing Northern Hemispheric ice sheets, show a dominant cycle of roughly 100 kyr period. The records also show smaller-amplitude oscillations with spectral peaks at roughly 40 and 20 kyr periods, which are well correlated with the Milankovich insolation variations due to perturbations in the Earth's orbital parameters. However, no model has accurately simulated the 100 kyr glacial cycle. Recently Birchfield et al.1 and Oerlemans2 have obtained encouraging agreement with some features of the glacial cycle by using a simple ice sheet model with a realistic time lag in the response of the bedrock to the ice load. This study extends their basic model, first by including topography to represent high ground in the north. Improved results can then be obtained but only with unrealistic parameter values and for some aspects of the record. Further improvements areobtained by crudely parameterizing possible calving at the equatorward ice sheet tip during deglaciation by proglacial lakes and/or marine incursions from the Atlantic, as emphasized by Andrews3. The resulting ice volume curves agree fairly well with the observed records and their power spectra over the past 700 kyr.
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