This is affected by solar activity and the earth’s magnetic field.
Luckily, we can measure these fluctuations in samples that are dated by other methods.
Radiocarbon dating works by comparing the three different isotopes of carbon.
Isotopes of a particular element have the same number of protons in their nucleus, but different numbers of neutrons.
The explanation was that the physicists had assumed that the amount of C-14 in the atmosphere had been constant, when in fact it had varied over time.
The solution came using dendrochronology (tree ring dating).
When this method was first developed, a fairly large amount of carbon was necessary for dating but use of the AMS (accelerator mass spectrometer) today necessitates only a few milligrams for analysis.
The Conversation UK receives funding from Hefce, Hefcw, SAGE, SFC, RCUK, The Nuffield Foundation, The Ogden Trust, The Royal Society, The Wellcome Trust, Esmée Fairbairn Foundation and The Alliance for Useful Evidence, as well as sixty five university members.Another problem derives from the “reservoir effect” in which old material, limestone or graphite, has contaminated the samples. This is particularly true of marine samples and contemporary shells may seem to be hundreds of years old. The uncalibrated date is given with the unit BP (radiocarbon years before 1950).The calibrated date is also presented, either in BC or AD or with the unit cal BP (calibrated before present - before 1950).