By measuring the ratio of the radio isotope to non-radioactive carbon, the amount of carbon-14 decay can be worked out, thereby giving an age for the specimen in question.
But that assumes that the amount of carbon-14 in the atmosphere was constant — any variation would speed up or slow down the clock.
Carbon normally occurs as Carbon-12, but radioactive Carbon-14 may sometimes be formed in the outer atmosphere as Nitrogen-14 undergoes cosmic ray bombardment.
The resulting C-14 is unstable and decays back to N-14 with a measured half-life of approximately 5,730 years.
Climate records from a Japanese lake are set to improve the accuracy of the dating technique, which could help to shed light on archaeological mysteries such as why Neanderthals became extinct.