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Of more recent date was the overcompensating effect of man-made carbon-14 injected into the atmosphere during nuclear bomb testing.
The result was a rise in the atmospheric carbon-14 level by more than 50 percent.
In this way, the deviations can be compensated for and the carbon-14 age of the sample converted to a much more precise date.
Calibration curves have been constructed using dendrochronological data (tree-ring measurements of bristlecone pines as old as 8,200 years); periglacial varve, or annual lake sediment, data (); and, in archaeological research, certain materials of historically established ages.
It is now clear that carbon-14 is not homogeneously distributed among today’s plants and animals.
Studies have revealed that the atmospheric radiocarbon level prior to 1000 it was about 8 percent above what it is today.
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In addition to spatial variations of the carbon-14 level, the question of temporal variation has received much study.
A 2 to 3 percent depression of the atmospheric radioactive-carbon level since 1900 was noted soon after Libby’s pioneering work, almost certainly the result of the dumping of huge volumes of carbon-14-free carbon dioxide into the air through smokestacks.