Research has been ongoing since the 1960s to determine what the proportion of in the atmosphere has been over the past fifty thousand years.
The resulting data, in the form of a calibration curve, is now used to convert a given measurement of radiocarbon in a sample into an estimate of the sample's calendar age.
It became clear that there had been significant changes in the Earth's topography over time and that these changes could neither be accounted for by natural processes operating during the brief nor by the postulated Noachian flood.
Dating is not necessary to demonstrate that evolution is a fact.
Chronological sequence is all that is really required.
Other corrections must be made to account for the proportion of throughout the biosphere (reservoir effects).
Additional complications come from the burning of fossil fuels such as coal and oil, and from the above-ground nuclear tests done in the 1950s and 1960s.