Snowball Bistability Vanishes at Moderate Orbital Eccentricity

Abstract: Snowball episodes are associated with increases in atmospheric oxygen and the complexity of life on Earth, and they may be essential for the development of complex life on exoplanets. Sustained stable Snowball episodes require a Snowball bifurcation and climate bistability between the globally ice-covered Snowball state and a state with at least some open ocean. We find that climate bistability disappears for an aquaplanet with a slab ocean in the Global Climate Model ExoCAM when the orbital eccentricity is increased to 0.2-0.3. This happens because the Snowball state loses stability as seasonal insolation variations intensify, while the warm state remains stable due to the ocean's large heat capacity. We use a low-order ice-thermodynamic model to show that the Snowball state loses stability as seasonality increases because winter freezing at the ice bottom is reduced relative to summer melting at the ice top due to ice self-insulation. Combined with previous research showing that Snowball climate bistability diminishes for planets orbiting low-mass stars, ones with longer rotation periods, and disappears entirely for tidally locked planets, our work suggests that the Snowball climate bistability may not be as robust to planetary parameters as previously thought, representing one aspect of habitability more consistent with the Rare Earth Hypothesis than the Copernican Principle.