Nuclear tests, nuclear reactors and the use of nuclear weapons have also changed the composition of radioisotopes in the air over the last few decades.
This human nuclear activity will make precise dating of fossils from our lifetime very difficult due to contamination of the normal radioisotope composition of the earth with addition artificially produced radioactive atoms.
When scientists first began to compare carbon dating data to data from tree rings, they found carbon dating provided "too-young" estimates of artifact age.
Carbon-14 cannot be used to date biological artifacts of organisms that did not get their carbon dioxide from the air.
This rules out carbon dating for most aquatic organisms, because they often obtain at least some of their carbon from dissolved carbonate rock.
Though still heavily used, relative dating is now augmented by several modern dating techniques.
Radiocarbon dating involves determining the age of an ancient fossil or specimen by measuring its carbon-14 content.
Carbon-14 has a half-life of 5,730 ± 40 years, meaning that every 5,700 years or so the object loses half its carbon-14.