Answer
The proportion of carbon-14/carbon-12 in the atmosphere has changed since 1900.
Work Step by Step
Humans have emitted a lot of carbon into the atmosphere since 1900, for example in the form of carbon dioxide.
The primary natural source of C-14 is cosmic ray action on nitrogen in the atmosphere. However, in the 1900s, humans carried out open-air nuclear testing, which also contributed to the amount of C-14.