The complication is that the concentration of carbon-14 in the atmosphere has not been constant over time.

It varies in response to changes in solar activity, the Earth’s magnetic field, and other factors.

Because the concentration was different in the past, a raw radiocarbon measurement does not directly correspond to the same number of calendar years.

Calibration corrects for this variation, using independent records of past atmospheric carbon-14 concentrations derived from tree rings, coral samples, cave deposits, and other archives.

The calibration process converts a raw radiocarbon age into a range of probable calendar years, with the uncertainty expressed as a statistical range within which the true calendar age is likely to fall.