Bomb pulse: increase in radioactive carbon-14 was produced by tests of nuclear bombs
Sudden increase of carbon-14 from nuclear tests up to 1963. The atmospheric level has since decayed exponentially.
Image source: wikipedia
Interestingly, the decay is not from radioctive decay, but because it gets absorbed away from the atmosphere. The amount of carbon-14 has increased in everything in earth, and the bomb pulse serves as a way to date things which is different from classical radiocarbon dating. It also affects classical radiocarbon directly, because carbon dating uses carbon-14, so this “pulse bomb” needs to be accounted for for carbon dating to work correctly.
More interesting details in the wikipedia article and in the BBC article “The atomic bomb marker inside your body”
Form the wikipedia article: approx 4% decay per year since 1963.
Starting from the pulse around the year 1963 (see figure), atmospheric radiocarbon relative abundance decreased by about 4% a year
I can also be expressed as difference of two exponentials $e^{-0.1t} - e^{-0.7t}$ (a surge).