High pCO2 Reduces Sensitivity to CO2 Perturbations on Temperate, Earth-Like Planets Throughout Most of Habitable Zone

The nearly logarithmic radiative impact of CO2 means that planets near the outer edge of the liquid water habitable zone (HZ) require similar to 10(6) x more CO2 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 CO2 changes of a given mass will have smaller temperature effects on higher pCO(2) planets. Ocean pH is linked to atmospheric pCO(2) through seawater carbonate speciation and calcium carbonate dissolution/precipitation, and the response of pH to changes in pCO(2) also decreases at higher initial pCO(2). Here, we use idealized climate and ocean chemistry models to demonstrate that CO2 perturbations large enough to cause catastrophic changes to surface temperature and ocean pH on temperate, low-pCO(2) planets in the innermost region of the HZ are likely to have much smaller effects on planets with higher pCO(2), 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 CO2 outgassing on high-pCO(2) planets in the mid-to-outer HZ should have mild or negligible impacts on surface temperature and ocean pH. Owing to low pCO(2), Phanerozoic Earth's surface environment may be unusually volatile compared with similar planets receiving lower instellation.

Automated summary

The impact of CO2 on planetary temperature and ocean pH decreases with increasing pCO(2), resulting in smaller effects on high pCO(2) planets. Major CO2 disturbances on high pCO(2) planets in the outer habitable zone are likely to have mild or negligible impacts on surface temperature and ocean pH. The surface environment of the Phanerozoic Earth, with low pCO(2), may be unusually volatile compared to similar planets receiving lower insolation.