Carbon cycle instability for high-$\mathrm{CO_2}$ exoplanets: Implications for habitability

Implicit in the definition of the classical circumstellar habitable zone (HZ) is the hypothesis that the carbonate-silicate cycle can maintain clement climates on exoplanets with land and surface water across a range of instellations by adjusting atmospheric $\mathrm{CO_2}$ partial pressure ($p\mathrm{CO_2}$). This hypothesis is made by analogy to the Earth system, but it is an open question whether silicate weathering can stabilize climate on planets in the outer reaches of the HZ, where instellations are lower than those received by even the Archean Earth and $\mathrm{CO_2}$ is thought likely to dominate atmospheres. Since weathering products are carried from land to ocean by the action of water, silicate weathering is intimately coupled to the hydrologic cycle, which intensifies with hotter temperatures under Earth-like conditions. Here, we use global climate model (GCM) simulations to demonstrate that the hydrologic cycle responds counterintuitively to changes in climate on planets with $\mathrm{CO_2}$-$\mathrm{H_2O}$ atmospheres at low instellations and high $p\mathrm{CO_2}$, with global evaporation and precipitation decreasing as $p\mathrm{CO_2}$ and temperatures increase at a given instellation. Within the MAC weathering formulation, weathering then decreases with increasing $p\mathrm{CO_2}$ for a range of instellations and $p\mathrm{CO_2}$ typical of the outer reaches of the HZ, resulting in an unstable carbon cycle that may lead to either runaway $\mathrm{CO_2}$ accumulation or depletion of $\mathrm{CO_2}$ to colder (possibly Snowball) conditions. While the behavior of the system has not been completely mapped out, the results suggest that silicate weathering could fail to maintain habitable conditions in the outer reaches of the nominal HZ.