EAPS

Title: The Snowball Bifurcation on Tidally Locked Planets

Abstract: The ice-albedo feedback on rapidly-rotating terrestrial planets in the habitable zone can lead to abrupt transitions (bifurcations) between a warm and a snowball (ice-covered) state, bistability between these states, and hysteresis in planetary climate. This is important for planetary habitability because snowball events
may trigger rises in the complexity of life, but could also endanger complex life that already exists. This raises the question of how the snowball bifurcation might work on synchronously rotating tidally locked planets, which have the same orbital period (year) and rotational period (day), in the habitable zone orbiting M and K dwarf stars. We investigate this question using analytical theory, an intermediate-complexity global climate model, and a coupled ocean-atmosphere global climate model. We find that planets locked in a 1:1 synchronous rotation state are likely to experience a smooth transition to global glaciation rather than a bifurcation. This is important because it means that tidally locked planets with an active silicate-weathering feedback loop will not stay in the snowball state. If they ever entered a snowball, they would quickly pop out of it because weathering would go to near zero while CO2 outgassing would continue.