If you’ve ever wondered about how archaeologists figure out how old a bone or other piece of ancient material is, you will be interested in this description of radiocarbon dating from EarthSky. Atmospheric chemists use the same principles to determine that most of the carbon dioxide in the atmosphere is not from recent burning but from the addition of carbon from fossil fuels millions of years old. It only works on things that were once alive, though, so they use different but similar techniques to date rocks and other non-living things. You can read about it here.
