Since the industrial revolution of the early nineteenth century, large amounts of fossil fuels (oil, coal, gas) have been combusted, causing an increase of about 10% in the concentration of atmospheric CO2. This added fossil CO2 was devoid of 14C and, correspondingly, lowered the 14C:12C ratio in the air by about 10%.
An anthropogenic addition of 14C into the atmosphere occurred with the nuclear bomb testing from 1952 to 1963, along with the introduction of bomb tritium. As a result, the 14C concentration also increased in plants (Fig. 11.2), in the soil CO2, and in recharged groundwater. Values up to 200 pmc have been measured, but they decreased to about 120 pmc by 1987, and to about 110 pmc by 1995.
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