This plot shows how the radiocarbon measurement 3000 -30BP would be calibrated.The left-hand axis shows radiocarbon concentration expressed in years `before present' and the bottom axis shows calendar years (derived from the tree ring data).

radiocarbon dating b p-25radiocarbon dating b p-3radiocarbon dating b p-13

The first indicates the proportion of radiocarbon atoms in the sample as compared to samples modern in 1950.

The second is directly derived from this on the assumption that the half-life of radiocarbon is 5568 years and the amount of radiocarbon in the atmosphere has been constant.

By using dead trees of different but overlapping ages, you can build up a library of tree rings of different calendar ages.

This has now been done for Bristlecone Pines in the U. A and waterlogged Oaks in Ireland and Germany, and Kauri in New Zealand to provide records extending back over the last 14,000 years.

This requires a computer since the calculations are more complicated.

It gives the time range, from which you can be 95% sure the sample came.

These values should correspond exactly to normal historical years BC and AD.

The term cal BP means the number of years before 1950 and can be directly compared to calendar years.

To extend this method further we must use the fact that tree ring widths vary from year to year with changing weather patterns.

By using these widths, it is possible to compare the tree rings in a dead tree to those in a tree that is still growing in the same region.

If we have a tree that is 500 years old we can measure the radiocarbon in the 500 rings and see what radiocarbon concentration corresponds to each calendar year.