It has been noted that several distinct ice ages existed during the past million years. Among other things, the marine sediment δ18O record, interpreted as the variation through time of the ice mass, indirectly confirms the Milankovitch hypothesis that orbital variation governs climatic changes, and leads to a great number of sophisticated models. A simplified feedback model for the global variations in time of ice volume, mean surface temperature and atmospheric carbon dioxide concentration is constructed. The transfer function of the system came from our differential equation system solved by using Laplace and Z-transform techniques. The system must conserve energy, the input being the incident solar energy and the output the energy radiated by the earth. The atmosphere and surface of the earth are assumed to be a fast responding climatic system, while the ocean and ice shell have a slower response. The difference in the time constants of the main reservoirs explains the phase shift between the input and the output of the system and its transient behaviour. As a hypothesis the glaciers have a non linear response. This simplified approach makes possible global energy transfer studies of the earth simultaneously including parameters such as the ice shell, the deep oceans, the atmosphere and surface, and the greenhouse effect. The ice mass over the last 800 K years computed by this approach, is in good agreement with the calculation using marine sediment δ18O. We suggest that the CO2 variation over this period of time could result from feedback effect (governed by orbital variations), rather than from a direct geological effect. The same approach could be used to investigate the variations of the mean surface temperature, the mean ocean temperature and CO2 atmospheric concentration.