Figure 1. The bidecadal (20-year averaged) tree ring record of the ¹⁴C/¹²C ratios for the last 11390 years (Stuiver and Reimer, 1993). Delta¹⁴C is the relative difference between the measured ¹⁴C/¹²C ratio corrected for decay and a fixed constant ¹⁴C/¹²C ratio (NBS Standard). The long-time increase of ¹⁴C/¹²C ratios going back in time is most likely due to a decrease of the geomagnetic field strength resulting in an increased ¹⁴C production rate. Shorter (~ 200-year) fluctuations are probably due to changes in the solar magnetic field which effect the penetration of galactic cosmic rays into the vicinity of the earth. However, variations in the CO₂ exchange rate between atmosphere and ocean (95 % of the global ¹⁴C inventory resides in the oceans) may also temporarily affect the atmospheric ¹⁴C/¹²C ratios. A detailed understanding of the causes for all the fluctuations is a challenging task for the future.