Fig. 1.

Atmospheric ¹⁴C content by year. The solid line represents background atmosphere Δ¹⁴C value since 1960, and the dashed and dotted lines represent atmosphere with FACE in elevated CO₂ plots at Aspen and Duke, respectively. Light gray lines represent the potential variability in ¹⁴C signature of the elevated CO₂ treatments based on the SD of CO₂ concentrations measured in elevated CO₂ plots (SI Methods).