Evolution of the mantle temperature (a) and the atmospheric CO₂ partial pressure (b) keeping the surface temperature constant at 300 K (solid lines) and 400 K (dashed lines). We use the CO₂ -dependent weathering steady-state model with α = 0.23 and initial mantle temperatures of 2000 K (red), 1900 K (yellow), and 1800 K (blue). The partial pressure decreases with time as more and more subduction zones become sufficiently cold so as to avoid decarbonation.