Use of radiocarbon dating
What you need is a ruler, a reliable map to the reservoir: in other words, an organic set of objects that you can securely pin a date on, measure its C14 content and thus establish the baseline reservoir in a given year.
Fortunately, we do have an organic object that tracks carbon in the atmosphere on a yearly basis: tree rings.
Reimer and colleagues point out that Int Cal13 is just the latest in calibration sets, and further refinements are to be expected.
For example, in Int Cal09's calibration, they discovered evidence that during the Younger Dryas (12,550-12,900 cal BP), there was a shutdown or at least a steep reduction of the North Atlantic Deep Water formation, which was surely a reflection of climate change; they had to throw out data for that period from the North Atlantic and use a different dataset.
Radiocarbon dating may only be used on organic materials.The half-life of an isotope like C14 is the time it takes for half of it to decay away: in C14, every 5,730 years, half of it is gone.So, if you measure the amount of C14 in a dead organism, you can figure out how long ago it stopped exchanging carbon with its atmosphere.The unstable nature of carbon 14 (with a precise half-life that makes it easy to measure) means it is ideal as an absolute dating method.The other two isotopes in comparison are more common than carbon-14 in the atmosphere but increase with the burning of fossil fuels making them less reliable for study (2); carbon-14 also increases, but its relative rarity means its increase is negligible. After this point, other Absolute Dating methods may be used.