Fig. 4.

Relationship between low-latitude PO₄ and pCO₂ in box model simulations. High-latitude export Φ_h is held constant at an intermediate value for the simulations shown here. The variable P:C simulations (orange) show a much larger impact of low-latitude PO₄ on pCO₂ than the fixed P:C simulations (gray). The enhanced sensitivity of the variable P:C simulations arises from the greater carbon storage achieved by low-latitude export, per mol of P export, compared with high-latitude export. The range of possible CO₂, corresponding to the 95% confidence intervals of the Line of Frugality regression shown in Fig. 1, are indicated by the region of orange shading.