  The nearly logarithmic radiative impact of CO$_2$ means that planets near the
outer edge of the liquid water habitable zone (HZ) require $\sim$10$^6$x more
CO$_2$ to maintain temperatures 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$_2$ 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 CO$_2$ perturbations
large enough to cause catastrophic changes to surface temperature and ocean pH
on low-pCO$_2$ planets in the innermost region of the HZ are likely to have
much smaller effects on planets with higher pCO$_2$. Major bouts of
extraterrestrial fossil fuel combustion or volcanic CO$_2$ 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 to similar
planets receiving lower instellation.
