GCM Simulations of Unstable Climates in the Habitable Zone

Abstract

It has recently been proposed that Earth-like planets in the outer regions of the habitable zone experience unstable climates, repeatedly cycling between glaciated and deglaciated climatic states (Menou 2015). While this result has been confirmed and also extended to explain early Mars climate records Batalha et al. 2016), all existing work relies on highly idealized low-dimensional climate models. Here, we confirm that the phenomenology of climate cycles remains in 3D Earth climate models with considerably more degrees of freedom. To circumvent the computational barrier of integrating climate on Gyr timescales, we implement a hybrid 0D-3D integrator which uses a general circulation model (GCM) as a short relaxation step along a long evolutionary climate sequence. We find that GCM climate cycles are qualitatively consistent with reported lowdimensional results. This establishes on a firmer ground the notion that outer habitable zone planets may be preferentially found in transiently glaciated states.

Habitable Snowballs: Temperate Land Conditions, Liquid Water, and Implications for CO2 Weathering

“…Continental weathering in transiently temperate land regions during snowball represents an additional fixed point, as shown in panel( b )of Figure , permitting stable solutions for low outgassing rates, represented here as all areas below the blue circles. Globally warm climates are stable for outgassing rates above the black dotted line, as found by Paradise and Menou (). Because the temperate snowball weathering limit and the globally warm weathering limit intersect, there is a region of parameter space (green) where both climates are stable for a given outgassing rate.…”

“…These climates sustain low to moderate weathering rates, implying potential for long‐term stability. Together with the potential for climate cycles consisting of long snowball events (Menou, ; Paradise & Menou, ), our results suggest that some fraction of observed terrestrial habitable‐zone planets may be snowball planets. This may be particularly important for older planets with lower outgassing as well as younger planets in the outer habitable zone, where snowball planets become likely even at Earth‐like outgassing rates.…”

“…represented here as all areas below the blue circles. Globally warm climates are stable for outgassing rates above the black dotted line, as found by Paradise and Menou (2017). Because the temperate snowball weathering limit and the globally warm weathering limit intersect, there is a region of parameter space (green) where both climates are stable for a given outgassing rate.…”

“…On planets with low outgassing rates, this equilibrium temperature may be colder than the transition to a snowball state (Pierrehumbert, 2005). Previous studies have proposed that a cessation of weathering due to globally cold conditions could mean that such planets slowly outgas enough CO 2 to eventually exit snowball, resulting in limit cycles (Haqq-Misra et al, 2016;Menou, 2015;Paradise & Menou, 2017). In other words, no stable point exists in low-outgassing regimes.…”

“…We implement weathering at each grid point following the technique in Paradise and Menou (2017), itself similar to techniques used in Le Hir et al (2009) and Edson et al (2012). In Paradise and Menou (2017), however, we parameterized precipitation rates in terms of surface temperature, rather than treating the role of precipitation directly. Here, we make use of the GCM's grid-level data for precipitation to include it directly.…”