The wood in these rings once laid down remains unchanged during the life of the tree.
This is very useful as a record of the radiocarbon concentration in the past.
To give an example if a sample is found to have a radiocarbon concentration exactly half of that for material which was modern in 1950 the radiocarbon measurement would be reported as 5568 BP.
For this purpose `present' refers to 1950 so you do not have to know the year in which the measurement was made.Generally, it is not possible to construct a complete sequence of tree rings back through the historical periods using only living trees.Chronologies derived from living trees must be extended.“The discovery of past spikes in atmospheric radiocarbon activity, caused by major solar energetic particle events, has opened up new possibilities for high-precision chronometry,” the paper said.Academics believe that powerful solar storms caused bursts of radiation that showered down on Earth in 775 to 994 CE.Radiocarbon, or Carbon-14, dating is probably one of the most widely used and best known absolute dating methods. Radiocarbon dating relies on a simple natural phenomenon.