When carbon-14 production was higher than average, organisms incorporated more carbon-14 into their tissues, which means that when those organisms are dated, their carbon-14 content looks like that of a younger organism — one that died more recently.

Conversely, when carbon-14 production was lower, organisms appear older than they are.

The calibration curves that correct for this variation are built from samples of known calendar age.

Tree rings are the most important source: trees produce one ring per year, and the carbon-14 content of each ring can be measured independently, creating a year-by-year record of atmospheric carbon-14 variation extending back tens of thousands of years.

Corals, cave speleothems, and other materials with independent calendar-year chronologies supplement the tree-ring record for periods beyond the range of the longest tree-ring series.