The above-ground nuclear tests that occurred in several countries between 1955 and 1980 (see nuclear test list) dramatically increased the amount of carbon-14 in the atmosphere and subsequently in the biosphere; after the tests ended, the atmospheric concentration of the isotope began to decrease, as radioactive  was fixed into plant and animal tissue, and dissolved in the oceans.
One side-effect of the change in atmospheric carbon-14 is that this has enabled some options (e.g., bomb-pulse dating) for determining the birth year of an individual, in particular, the amount of carbon-14 in tooth enamel, or the carbon-14 concentration in the lens of the eye.
In 2019, Scientific American reported that carbon-14 from nuclear bomb testing has been found in the bodies of aquatic animals found in one of the most inaccessible regions of the earth, the Mariana Trench in the Pacific Ocean.
The concentration of carbon-14 in atmospheric CO, reported as the ratio of carbon-14 to carbon-12 with respect to a standard,  has now (approximately since the year 2022) declined to levels similar to those prior to the above-ground nuclear tests of the 1950s and 1960s. Although the extra carbon-14 atoms generated during those nuclear tests have not disappeared from the atmosphere, oceans and biosphere, they are diluted because of the Suess effect.