We have used a recent estimate of the geomagnetic dipole field strength over the lasg 80 ky to examine the extent to which changes in geomagnetic field intensity have affected the radiocarbon time scale. The magnetic record was converted into a 14C production signal which was then used to produce a synthetic atmospheric Δ14C record using a two box model for the carbon cycle. This calibration suggests that the geomagnetic field has been the main factor governing the production of cosmogenic 14C atoms over the period of time considered, and that Δ14C changes due to other causes, such as climatic changes, have been of lesser importance. The results indicate that the radiocarbon ages are 2 to 3 ky younger than true age in the interval 18--40 ky b.p., in good agreement with recent U-Th and 14C age comparisons. Moreover the results predict that a negligible difference between true and 14C ages should be observed around 45--50 ky.