Modelling northern hemisphere ice volume over the last 3 Ma

The Northern Hemisphere ice-sheet volumes over the last 3 Ma were simulated using the LLN 2-D model. The forcings were both insolation and CO2 concentration. Different atmospheric CO2 scenarios were used because of a lack of CO2 reconstruction over this remote past. With constant CO2 concentrations, the simulated ice volume does not show any gradual increase as recorded in the marine sediments. Moreover, its spectrum changes according to the chosen value of CO2: when CO2 is fixed at 220 ppmv, the simulated ice volume is dominated by the similar to 100 ka period; if a pre-industrial CO2 concentration of 280 ppmv is used, the simulation is dominated by the similar to 41 ka period. By using a linearly decreasing CO2 concentration going from 320 ppmv at 3 Ma BP to 200 ppmv at present, the simulated changes in the power spectrum are in agreement with those obtained from the sedimentary records: roughly before 1 Ma BP the dominating period is similar to 41 ka, afterwards the similar to 100 ka period becomes dominant. This transition is accompanied by a gradual increase of the ice volume. Potential mechanisms for this transition are discussed. Using cyclic CO2 fluctuations over the last 0.6 Ma does not change significantly the spectral characteristics of the simulated ice volume, but amplifies the amplitude of its variations. (C) 1998 Elsevier Science Ltd. All rights reserved.