The most recent calculations of the infra-red flux in the region of the 15 micron CO2 band show that the average surface temperature of the earth increases 3.6° C if the CO2 concentration in the atmosphere is doubled and decreases 3.8° C if the CO2 amount is halved, provided that no other factors change which influence the radiation balance. Variations in CO2 amount of this magnitude must have occurred during geological history; the resulting temperature changes were sufficiently large to influence the climate. The CO2 balance is discussed. The CO2 equilibrium between atmosphere and oceans is calculated with and without CaCO3 equilibrium, assuming that the average temperature changes with the CO2 concentration by the amount predicted by the CO2 theory. When the total CO2 is reduced below a critical value, it is found that the climate continuously oscillates between a glacial and an inter-glacial stage with a period of tens of thousands of years; there is no possible stable state for the climate. Simple explanations are provided by the CO2 theory for the increased precipitation at the onset of a glacial period, the time lag of millions of years between periods of mountain building and the ensuing glaciation, and the severe glaciation at the end of the Carboniferous. The extra CO2 released into the atmosphere by industrial processes and other human activities may have caused the temperature rise during the present century. In contrast with other theories of climate, the CO2 theory predicts that this warming trend will continue, at least for several centuries.