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It has since decreased to about 10% above "pre-bomb" levels, due mainly to the exchange of CO2 between the atmosphere and the oceans. Radiocarbon ages are conventionally specified relative to the year 1950, defined as "present". This is accomplished by comparing the measured CRA with a table of radiocarbon ages measured on samples of wood whose chronological ages are known exactly by tree-ring counting.
Tables of tree-ring data going back 12,000 years are available (see Radiocarbon Calibration Issue, 1993), and work to refine the calibration data is continuing.