A unit used in stating the nominal ages of plant or animal remains dated by radiocarbon testing. A very small proportion (roughly 1 part per trillion, or 10^-12) of the carbon in the ecosystem is radioactive carbon-14, which decays to nitrogen-14 with a half life of about 5760 years. While a plant or animal is alive, the fraction of radioactive carbon in its body remains equal to the fraction in the atmosphere at that time. After a plant or animal dies, as the carbon-14 trapped in its body slowly decays, the age of the tissue can be measured by the fraction of radioactive carbon remaining. The age T of a sample, in radiocarbon years, is computed from the formula T = -8033 ln (R/A), where R is the measured ratio of carbon-14 to ordinary carbon-12 in the sample and A is a benchmark ratio measured in the atmosphere in 1950. Results are often stated as years before present (yr BP), with 1950 chosen to be the "present." The results are inaccurate for several reasons. Obviously, 1950 is no longer the present. Another problem is that the formula assumes a half life of 5568 years, which is now known to be too short (the actual value is 5730±40 years). Most importantly, the ratio of carbon-14 to ordinary carbon-12 in the atmosphere varies slightly over time. Much research has been done to determine the necessary corrections. As an example, a sample with a nominal age of 12 000 radiocarbon years has an actual age of about 14 000 years. A table is provided, and a technical report (pdf document) posted by the University of Arizona has full details