## 2.31.4. 14C radioactive decay and historical atmospheric 14C and 13C concentrations[¶](#c-radioactive-decay-and-historical-atmospheric-14c-and-13c-concentrations "Permalink to this headline")
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In the preindustrial biosphere, radioactive decay of 14C in carbon pools allows dating of long-term age since photosynthetic uptake; while over the 20\\({}^{th}\\) century, radiocarbon in the atmosphere was first diluted by radiocarbon-free fossil fuels and then enriched by aboveground thermonuclear testing to approximately double its long-term mean concentration. CLM includes both of these processes to allow comparison of carbon that may vary on multiple timescales with observed values.

For radioactive decay, at each timestep all 14C pools are reduced at a rate of –log/\\(\\tau\\), where \\(\\tau\\) is the half-life (Libby half-life value of 5568 years). In order to rapidly equilibrate the long-lived pools during accelerated decomposition spinup, the radioactive decay of the accelerated pools is also accelerated by the same degree as the decomposition, such that the 14C value of these pools is in equilibrium when taken out of the spinup mode.

For variation of atmospheric 14C and 13C over the historical period, \\(\\mathrm{\\Delta}\\)14C and \\(\\mathrm{\\Delta}\\)13C values can be set to either fixed concentrations or time-varying concentrations read in from a file. A default file is provided that spans the historical period ([Graven et al., 2017](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#gravenetal2017)). For \\(\\mathrm{\\Delta}\\)14C, values are provided and read in for three latitude bands (30°N–90°N, 30°S–30°N, and 30°S–90°S).
