By comparing the activity of the Carbon-14 and the equilibrium levels of living organism a time can be found for this method to work.
In dead plants or animals, however, the carbon-14 decays with a half-life of 5,730 years.
For practical dating purposes, measurements of carbon-14 are adjusted to match the tree-ring data, so as to compensate for small changes in the amount of atmospheric carbon-14 over time.
AD 1963 marked the signing of the Partial Test Ban Treaty (PTBT) by the United States, United Kingdom, and the Soviet Union wherein it was agreed that nuclear weapons testing would no longer be performed above ground surface.
Since then, the global radiocarbon level of the atmosphere has decreased through uptake in the oceans and biological systems – e.g.
The period of time that it takes for half of a sample to decay is called a "half-life." Radiocarbon oxidizes (that is, it combines with oxygen) and enters the biosphere through natural processes like breathing and eating.