Fig. 6: The expected distribution of stable, steady-state pCO2 on Earth-like planets if Q, fland, and fbio are fixed to 1, i.e., modern Earth values.

Except for fixing Q = fland = fbio = 1, this figure is generated identically to Fig. 1. The spread in pCO2 in the outer HZ is due to the temperature- and pCO2-dependence of continental weathering (Te and α). This is expected from Eq. (4), which shows that Te and α will be increasingly influential as pCO2 and surface temperature deviate from the modern Earth values, as discussed in the Results, subsection “Habitable zone climate theory revisited”. Without changes in Q, fland, and fbio, there is little spread in pCO2 in the inner HZ.