How likely are Snowball episodes near the inner edge of the habitable zone?

TL;DR

This study uses a simple stochastic model to explore how increasing stellar luminosity affects climate stability on Earth-like planets, suggesting frequent Snowball states near the habitable zone's inner edge.

Contribution

It introduces a stochastic CO2 and climate model to analyze the likelihood of Snowball episodes, highlighting the importance of climate feedbacks in planetary habitability.

Findings

Increased stellar luminosity leads to climate instability.

Planets near the habitable zone's inner edge may frequently enter Snowball states.

Future observations can test these predictions.

Abstract

Understanding when global glaciations occur on Earth-like planets is a major challenge in climate evolution research. Most models of how greenhouse gases like CO2 evolve with time on terrestrial planets are deterministic, but the complex, nonlinear nature of Earth's climate history motivates study of non-deterministic climate models. Here a maximally simple stochastic model of CO2 evolution and climate on an Earth-like planet with an imperfect CO2 thermostat is investigated. It is shown that as stellar luminosity is increased in this model, the decrease in the average atmospheric CO concentration renders the climate increasingly unstable, with excursions to a low-temperature state common once the received stellar flux approaches that of present-day Earth. Unless climate feedbacks always force the variance in CO2 concentration to decline rapidly with received stellar flux, this means that terrestrial planets near the inner edge of the habitable zone may enter Snowball states quite frequently. Observations of the albedos and color variation of terrestrial-type exoplanets should allow this prediction to be tested directly in the future.