"When we're going back in time, you don't have that effect, it's really the natural radiocarbon you're using to get the measurements from."

The best prior separation method was based on the correlation between natural radiocarbon and dissolved silicate.

But even before the "bomb carbon" period, in the first half of the 20th Century, humans were already disrupting the natural radiocarbon signal by burning fossil fuels such as oil, coal and gas.