[eigenket]

This graph [1] available on Wikipedia answers this question. The level of carbon 14 in the atmosphere (in the southern hemisphere) roughly doubled between 1955 and 1963 ish. This coincides with the era of above ground nuclear testing. Since then it has been decaying back to the baseline.

[1] https://en.wikipedia.org/wiki/Carbon-14#/media/File%3ARadioc...

[prerok]

Indeed, I may have misread the GP comment, understanding that it stated that C-14 appeared because of the nuclear tests. They may have meant the addition. I did try to correct my mistake by answering my own question...