THE INITIATION OF ICE-SHEET GROWTH, MILANKOVITCH SOLAR-RADIATION VARIATIONS, AND THE 100 KY ICE-AGE CYCLE

Much work has gone into deciphering the causes of the large scale glac ial/interglacial variations in the climate system over the last 900 00 0 years. While variations on the 41 thousand year (ky) and 23 ky time scales seem to be linearly linked to the variations in the distributio n of solar radiation at the top of the atmosphere, Milankovitch solar radiation variations, the causes of the dominant 100 ky cycle in the g eologic record are still unknown. One of the aspects of this cycle tha t is not well understood is how large scale ice sheet growth is initia ted. Here we describe the mechanisms by which large scale ice sheet gr owth may have been initiated by the changes in the seasonal and latitu dinal distribution of solar radiation over the past 160 ky. This is do ne through the use of a coupled energy balance climate-thermodynamic s ea ice model that includes a hydrologic cycle which computes precipita tion, and a land surface energy balance which determines the net accum ulation of snow and ice. Results indicate that the initiation of ice s heet growth is possible during times of extremely low summer solstice solar radiation as a result of a large decrease in ablation during the critical melt season.