Fig. 6: Observed versus predicted δ¹³CO₂ (‰) estimated from a simple carbon cycle model with three biosphere boxes.

a Original model configuration predicting δ¹³CO₂ with simple (blue) and CO₂-driven (green) Δ¹³C as in ref. ³ and ref. ³⁰—denoted Keeling2017 and Graven2020, respectively. b Model predictions when box 1 represents only C₄ plants, while boxes 2 and 3 are populated only by C₃ plants. In blue is the simulation when both Δ¹³C and F₄ are constant. In green is the prediction when Δ¹³C is modeled as in Equations S11–S13 with constant F₄ and F₃. In brown is the simulation when Δ¹³C, F₄, and F₃ vary.