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(You can read up on radioactivity and isotopes here).Carbon-14, the radioactive version of carbon, is rare — it only makes up one trillionth of all the carbon in the world.And nuclear reactions have seen a leap in carbon-14 activity since 1945.Luckily for us we have a record of atmospheric carbon-14 levels for every one of the last 12,000 years.And after 11,460 years (two half-lives), only a quarter of the original carbon-14 atoms are left.
The level of radiocarbon in the atmosphere has varied over time — it was about two per cent higher 3,500 years ago, possibly due to factors affecting cosmic rays (like changes in solar cycles or the Earth's magnetic field).
For a rare event it happens pretty damn often — one million carbon-14 atoms in your body decay into nitrogen every minute!
But don't panic — of the 800,000,000,000,000,000,000,000,000 carbon atoms in every one of us, about 800,000,000,000,000 are carbon-14, so we've got a few to spare.
So calculating the age range of a once-living sample involves measuring the 14C/12C ratio, and using this the known half-life to estimate the length of time since the sample died.
That age range is then compared with known 14C/12C ratios from the tree ring/marine record to find the best match, and the result is a calibrated age range you can be 95 per cent sure of.