Calibration of radiocarbon results is needed to account for changes in the atmospheric concentration of carbon-14 over time.These changes were brought about by several factors including, but not limited to, fluctuations in the earth’s geomagnetic moment, fossil fuel burning, and nuclear testing.The most common isotope is carbon-12 (or 12 C), which (according the article) makes up 98.89 percent of the naturally occurring carbon.There's carbon-13, or 13 C, which is much rarer, accounting for only 1.11 percent, and then there's carbon-14, or 14 C, which makes up a ridiculously tiny fraction of existing carbon. Living organic matter will have steady and predictable concentrations of each isotope of carbon, pretty much the percentages mentioned above. After something dies, the 14 C decays over time (because it is radioactive) and doesn't replenish as it would in a live specimen because the dead thing isn't eating and breathing or otherwise exchanging molecules with the outside world anymore).For example, a particular object that has been dated might a radiocarbon age of 4500 years, plus or minus 30 years.The margin of error depends on the object, but for samples younger than 10,000 years, the uncertainty is usually at most 40 years.Although it is less accurate, the Libby half-life was retained to avoid inconsistencies or errors when comparing carbon-14 test results that were produced before and after the Cambridge half-life was derived.
The reason that carbon dating isn't exact is due to two reasons.
Carbon-14 is a naturally occurring isotope of the element carbon.
It is also called “radiocarbon” because it is unstable and radioactive relative to carbon-12 and carbon-13.
Scientists can measure the amount of carbon-14 in a piece of old wood for instance, and say that because there is only a certain amount left, the tree died 1000 years ago.
Calibration is not only done before an analysis but also on analytical results as in the case of radiocarbon dating—an analytical method that identifies the age of a material that once formed part of the biosphere by determining its carbon-14 content and tracing its age by its radioactive decay.