High pCO Reduces Sensitivity to CO Perturbations on Temperate, Earth-like Planets Throughout Most of Habitable Zone.

The nearly logarithmic radiative impact of CO means that planets near the outer edge of the liquid water habitable zone (HZ) require ∼10 × more CO to maintain temperatures that are conducive to standing liquid water on the planetary surface than their counterparts near the inner edge. This logarithmic radiative response also means that atmospheric CO changes of a given mass will have smaller temperature effects on higher pCO planets. Ocean pH is linked to atmospheric pCO through seawater carbonate speciation and calcium carbonate dissolution/precipitation, and the response of pH to changes in pCO also decreases at higher initial pCO. Here, we use idealized climate and ocean chemistry models to demonstrate that CO perturbations large enough to cause catastrophic changes to surface temperature and ocean pH on temperate, low-pCO planets in the innermost region of the HZ are likely to have much smaller effects on planets with higher pCO, as may be the case for terrestrial planets with active carbonate-silicate cycles receiving less instellation than the Earth. Major bouts of extraterrestrial fossil fuel combustion or volcanic CO outgassing on high-pCO planets in the mid-to-outer HZ should have mild or negligible impacts on surface temperature and ocean pH. Owing to low pCO, Phanerozoic Earth's surface environment may be unusually volatile compared with similar planets receiving lower instellation.