Figure 1. The ¹⁴C bomb curve is recorded in biomolecules. Aboveground nuclear testing nearly doubled the level of radiocarbon (as ¹⁴CO₂) in the atmosphere between 1955 and 1963. The atmospheric ¹⁴C levels depicted by the dark blue trace are growing season averages for the northern hemisphere expressed in F¹⁴C units (fraction modern with δ¹³C fractionation correction). These ¹⁴C levels are recorded in annual plant growth and human diets. Human tissue incorporates the contemporary ¹⁴C signature of individuals’ food at the time of synthesis. Years later, specific biomolecules can be isolated and measured for ¹⁴C content to establish carbon turnover or lack thereof, as in the cellulose of tree rings.