Sensitivity Experiments with a Model of the Ice Age Cycle: The Response to Harmonic Forcing

Abstract

It has been established beyond reasonable doubt that late Pleistocene ice volume fluctuations have been dominated by an oscillation of characteristic period near 100 000 years. The astronomical theory of ice ages asserts that all such ice volume fluctuations are driven by variations in effective solar constant. At least as conventionally defined, however, the strength of the astronomical forcing at the dominant 10⁵ year period is negligible. In an earlier paper we showed that a climate model including an accurate representation of the glacial isostatic adjustment process could produce ice volume fluctuations with 100 000-year periodicity from input with 20 000-year periodicity. This is important because 20 kyr is near the precessional periods which dominate the astronomical forcing below 60° north latitude. In this paper the 10⁵ year cycle in our model is examined in more detail, and its sharp terminations are shown to be due to the formation of a gravitational potential well which causes rapid flow into the ablation zone during the retreat of large ice sheets. The meteorological parameters of the climate model are varied in a new set of sensitivity experiments which demonstrate that the nature of ice sheet oscillations appears to be fairly stable against such variations.