Figure 1: Atmospheric ¹⁴CO₂ Concentrations Expressed in F¹⁴C

The graph includes annual averages from 1940 to 1954 of samples harvested from a Douglas Fir on the Olympic Peninsula (Stuiver et al. 1998b), monthly averages for the mid-latitudes of the Northern Hemisphere (Hua and Barbetti 2004), and approximately biweekly sampling of sites in Europe (Levin and Kromer 2004).

Traditionally, ¹⁴C data are reported as ¹³C fractionation corrected and ¹⁴C decay corrected (Δ¹⁴C), following the dominant convention of Stuiver and Polach (1977). Established for reporting radiocarbon data in chronological and geophysical studies, this convention was not developed to deal with postbomb data, but it is the most common, pending the adoption of the proposed standard F¹⁴C nomenclature for postbomb data (Reimer et al. 2004). Fraction modern (FM) and percent modern carbon (pMC) are sometimes used to report data, but they are not defined consistently in the literature. F¹⁴C is essentially fraction modern carbon with a ¹³C fractionation correction, where modern carbon (F¹⁴C = 1) represents a two-decade average of the atmospheric ¹⁴C level recorded in tree rings prior to the Industrial Revolution.