Radiocarbon has been used to define parameters for modeling past, recent, and future CO2/carbon amounts in the atmosphere and in other environmental compartments. In the present paper, we estimate the amount of 14C in the atmosphere by calculating the molar activity of 14CO2 (quantity of 14CO2 molecules per mol of air). Data on the reconstruction of the past concentration of atmospheric CO2 from Antarctic ice cores and Δ14C activities from the IntCal09 calibration curve were applied. The results obtained indicate that cosmogenic production had a dominant influence on the 14C amount in the atmosphere between 50 and 20 ka BP, when the CO2 concentrations were relatively stable, with a slowly decreasing trend. The decreasing 14C activity (Δ14C) between 20 and 2 ka BP seems to be caused predominantly by a dilution of atmospheric 14CO2 by input of CO2 with a depleted amount of 14C (probably from deeper oceanic layers), which is evident from a comparison with the Δ14C and molar activity time series. A strong linear relation was found between the 14C activity and CO2 concentration in the air for the period 20–2 ka BP, which confirms a dominant influence of atmospheric dilution of 14CO2. The observed linear relation between the CO2 and Δ14C levels persists even in the prevailing part of the Holocene. Likewise, the quantity of 14CO2 in the atmosphere (calculated as molar activity) during the prevailing part of the deglacial period (20–11 ka BP) was surprisingly increasing, although a decreasing trend in the 14C cosmogenic production rate could be expected.