A dating method for organic material that is applicable to about the last 70 000 years. It relies on the assumed constancy over time of atmospheric ¹⁴C:¹²C ratios (now known not to be valid), and the known rate of decay of radioactive carbon, of which half is lost in a period (the ‘half-life’) of every 5730 ± 30 years. (The earlier ‘Libby standard’, 5568 years, is still widely used.) In principle, since plants and animals exchange carbon dioxide with the atmosphere constantly, the ¹⁴C content of their bodies when alive is a function of the radiocarbon content of the atmosphere. When an organism dies, this exchange ceases and the radiocarbon fixed in the organism decays at the known half-life rate. Comparison of residual ¹⁴C activity in fossil organic material with modern standards enables the age of the samples to be calculated. Since the method was first devised it has been realized that the atmospheric ¹⁴C content varies, as the cosmic-ray bombardment of the outer atmosphere that generates the ¹⁴C varies. Correction for these fluctuations is possible for about the last 8000 years by reference to the ¹⁴C contents of long tree-ring series, e.g. those for bristlecone pine (Pinus longaeva).