The New Zealand curve is representative for the Southern Hemisphere, the Austrian curve is representative for the Northern Hemisphere. This makes it possible to tell the age of substances that contain carbon. Dates obtained are usually written as before present ('present' is 1950).
Bottom line: Radiocarbon dating is a technique used by scientists to learn the ages of biological specimens from the distant past.
Carbon has different isotopes, which are usually not radioactive; C is the radioactive one, its half-life, or time it takes to radioactively decay to one half its original amount, is about 5,730 years.
The relatively short-lived C taken into organic matter is also slightly variable. However, under about 20,000 years the results can be compared with dendrochronology, based on tree rings.
The unstable carbon-14 gradually decays to carbon-12 at a steady rate. Scientists measure the ratio of carbon isotopes to be able to estimate how far back in time a biological sample was active or alive.
This plot shows the level of carbon-14 in the atmosphere as measured in New Zealand (red) and Austria (green), representing the Southern and Northern Hemispheres, respectively.
For the most accurate work, variations are compensated by means of calibration curves.