The amount of C¹⁴ in the atomsphere varies over time

Atmospheric ¹⁴C, New Zealand and Austria. The New Zealand curve is representative for the Southern Hemisphere, the Austrian curve is representative for the Northern Hemisphere. Atmospheric nuclear weapon tests almost doubled the concentration of ¹⁴C in the Northern Hemisphere.

Radiocarbon dating, also known as the C14 dating method, is a way of telling how old an object is. It is a type of radiometric dating.

The method uses the radioactive isotope carbon-14. Most organic matter contains carbon. Carbon has different isotopes, which are usually not radioactive. ¹⁴C is the radioactive one, its half-life (time it takes to reduce its radioactivity by half) is about 5,730 years. This makes it possible to tell the age of substances that contain carbon. The method works to about 60,000 years old. Dates obtained are usually written as before present ('present' is 1950).