A thermodynamic climate model is used to simulate the Northern Hemisphere climates for 18, 13, 10, 7 and 4 kyr BP, and to evaluate the importance of the ice sheets, the insolation anomalies and the variation of the atmospheric CO2 in the maintainance and evolution of the terrestrial climates from 18 kyr BP to present time. The computations show the great importance of the insolation anomalies, due to orbital variations, in the annual cycle of climate, especially for the most recent period from about 12 kyr BP to present time, when the computed monthly average Northern Hemisphere surface temperature anomalies have the pattern of negative and positive anomalies shown in the insolation anomalies, except for a lag associated with the storage of heat in the oceans. It is shown that from 18 to 12 kyr BP the surface albedo feedback effect, due to the presence of ice sheets, maintains the average Northern Hemisphere surface temperature anomalies negative through the whole year, having its maximum effect at 18 kyr BP. The effect of the orbital variation is to produce larger seasonal variations in the surface temperatures, associated with the variations of the insolation anomalies which are non-negligible despite the fact that their annual average is negligible. The effect of the decrease of the atmospheric CO2 is to reinforce the negative anomalies of the surface temperature. The average of the computed surface ocean temperature anomalies for 18 kyr BP, is about two thirds of the value estimated by CLIMAP (1976), and is due to the combined effect of the existance of the ice sheets, the decrease of atmospheric CO2 and the anomalies of insolation, as external forcings.