Half life and radiocarbon dating
The level is affected by variations in the cosmic ray intensity which is affected by variations in the earth's magnetosphere caused by solar storms.
In addition there are substantial reservoirs of carbon in organic matter, the ocean, ocean sediments (see methane hydrate), and sedimentary rocks.
Plants fix atmospheric carbon during photosynthesis, so the level of C14 in living plants and animals equals the level of C14 in the atmosphere. Carbon-14 has a half-life of 5730 years and would have long ago vanished from Earth were it not for the unremitting cosmic ray impacts on nitrogen in the Earth's atmosphere, which create more of the isotope.
If there is little carbon-14 to begin with, a half-life that long means that very few of the atoms will decay while their detection is attempted (4 atoms/s) /mol just after death, hence e.g. Sensitivity has since been greatly increased by the use of accelerator-based mass-spectrometric (AMS) techniques, where all the C atoms can be counted directly, rather than only those decaying during the counting interval allotted for each analysis.
By emitting an electron and an anti-neutrino, carbon-14 is changed into stable (non-radioactive) nitrogen-14.