Carbon dating or radiocarbon
Carbon normally occurs as Carbon-12, but radioactive Carbon-14 may sometimes be formed in the outer atmosphere as Nitrogen-14 undergoes cosmic ray bombardment.
The resulting C-14 is unstable and decays back to N-14 with a measured half-life of approximately 5,730 years.
In reality, its measured disequilibrium points to just such a world-altering event, not many years ago.
It subsequently evolved into the most powerful method of dating late Pleistocene and Holocene artifacts and geologic events up to about 50,000 years in age.
The radiocarbon method is applied in many different scientific fields, including archeology, geology, oceanography, hydrology, atmospheric science, and paleoclimatology.
Thus the ratio of stable C-12 to unstable C-14, which is known in today's open environment, changes over time in an isolated specimen. As long as the tree lives, it absorbs carbon from the atmosphere in the form of carbon dioxide, both C-12 and C-14.
Once the tree dies, it ceases to take in new carbon, and any C-14 present begins to decay.