Radiocarbon dating exploits the radioactive decay of carbon-14, a naturally occurring isotope of carbon that is continuously produced in the upper atmosphere by cosmic ray bombardment. Living organisms constantly exchange carbon with their environment through eating and breathing, so while alive they maintain a constant ratio of carbon-14 to stable carbon isotopes (carbon-12 and carbon-13). When an organism dies, the exchange stops and the carbon-14 begins to decay at a known rate — its half-life is approximately 5,730 years. By measuring how much carbon-14 remains in a sample relative to what was originally present, the time elapsed since death can be calculated.
For a specimen like Cheddar Man, whose age falls around ten thousand years, approximately one and three-quarter half-lives of carbon-14 have elapsed, leaving around 29% of the original carbon-14. This is readily measurable with modern accelerator mass spectrometry, which can date samples containing as little as a milligram of carbon.
The Natural History Museum has noted that radiocarbon dating was performed on Cheddar Man twice, with consistent results confirming an age of approximately 10,000 years before present. The consistency across independent measurements is important: it reduces the probability that a single result was affected by contamination or laboratory error.
Radiocarbon ages must be calibrated to convert them to calendar years, because the concentration of carbon-14 in the atmosphere has varied through time due to changes in solar activity, ocean circulation, and other factors. Using calibration curves developed from tree-ring chronologies and other independently dated archives, radiocarbon ages around 10,000 years before present translate to approximately 8,000 BCE in calendar years — placing Cheddar Man in the early part of what archaeologists call the Boreal period, a phase characterized by warming temperatures, expanding forest cover, and rising sea levels as the ice sheets continued to melt.