How Likely Are Snowball Episodes Near The Inner Edge Of The Habitable Zone?

Output of the numerical stochastic model over 500 My for an ensemble of 1024 runs, given fixed stellar luminosity (α = 0.98), relaxation timescale τ = 2.5 My, and σy = 0.7. a) CO2 molar concentration vs. time, b) normalized histogram of CO2 molar concentration values, c) temperature vs. time and d) normalized histogram of temperature values. For a) and c), a single run where temperature dropped below the Snowball limit is shown in black. In c) the light blue shading indicates the Snowball transition region, while in both a) and c), the green line indicates the mean value. Finally, in b) and d) the solid black and red dashed lines indicate the numerical results and analytic results according to (6) and (14), respectively. The ice-albedo feedback is not included in these simulations.

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.

R. Wordsworth
Comments: 8 pages, 3 figures, under review at ApJ Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2104.06216 [astro-ph.EP] (or arXiv:2104.06216v1 [astro-ph.EP] for this version)
Submission history
From: Robin Wordsworth
[v1] Tue, 13 Apr 2021 14:10:17 UTC (1,293 KB)

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