Habitable Zones & Global Climate

Carbon Cycle Instability For High-CO2 Exoplanets: Implications For Habitability

By Keith Cowing

Status Report

astro-ph.EP

May 10, 2024

Filed under Archean Earth, astro-ph.EP, astrochemistry, atmosphere, carbon cycle, Circumstellar Habitable Zone, exoplanet, geochemistry, Global Climate Models, Habitable Zone

Carbon Cycle Instability For High-CO2 Exoplanets: Implications For Habitability — A schematic of the saddle node bifurcation in the carbon cycle suggested by our simulations. The blue curve represents the global weathering rate as a function of pCO2. Weathering increases with increasing pCO2 (and surface temperature) in the green zone, and it decreases with increasing pCO2 (and surface temperature) in the red zone. Weathering rates are equal to outgassing (V ) at pCO2=P1 (orange) and at pCO2=P2 (black). The P1 equilibrium is stable because weathering acts as a negative feedback in that regime, while the P2 equilibrium is unstable due to silicate weathering’s positive feedback behavior. The red arrows indicate the directions CO2 and weathering would evolve for a climate intialized from various points relative to the two equilibria. — astro-ph.EP

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 CO₂ partial pressure (pCO₂).

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 CO₂ 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 CO₂-H₂O atmospheres at low instellations and high pCO2, with global evaporation and precipitation decreasing as pCO₂ and temperatures increase at a given instellation.

Within the MAC weathering formulation, weathering then decreases with increasing pCO₂ for a range of instellations and pCO₂ typical of the outer reaches of the HZ, resulting in an unstable carbon cycle that may lead to either runaway CO₂ accumulation or depletion of CO₂ 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.

R.J. Graham, R.T. Pierrehumbert

Comments: Accepted for publication in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2405.05396 [astro-ph.EP] (or arXiv:2405.05396v1 [astro-ph.EP] for this version)
Related DOI:
https://doi.org/10.3847/1538-4357/ad45fb
Focus to learn more
Submission history
From: R.J. Graham
[v1] Wed, 8 May 2024 19:55:13 UTC (700 KB)
https://arxiv.org/abs/2405.05396
Astrobiology

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

Follow on Twitter