Abstract
Abstract. We study an Earth-like terra-planet (water-limited terrestrial planet) with an overland recycling mechanism bringing fresh water back from the high latitudes to the low latitudes. By performing model simulations for such a planet we find two drastically different climate states for the same set of boundary conditions and parameter values: a cold and wet (CW) state with dominant low-latitude precipitation and a hot and dry (HD) state with only high-latitude precipitation. We notice that for perpetual equinox conditions, both climate states are stable below a certain threshold value of background soil albedo while above the threshold only the CW state is stable. Starting from the HD state and increasing background soil albedo above the threshold causes an abrupt shift from the HD state to the CW state resulting in a sudden cooling of about 35 ∘C globally, which is of the order of the temperature difference between present day and the Snowball Earth state. When albedo starting from the CW state is reduced down to zero the terra-planet does not shift back to the HD state (no closed hysteresis). This is due to the high cloud cover in the CW state hiding the surface from solar irradiation so that surface albedo has only a minor effect on the top of the atmosphere radiation balance. Additional simulations with present-day Earth's obliquity all lead to the CW state, suggesting a similar abrupt transition from the HD state to the CW state when increasing obliquity from zero. Our study also has implications for the habitability of Earth-like terra-planets. At the inner edge of the habitable zone, the higher cloud cover in the CW state cools the planet and may prevent the onset of a runaway greenhouse state. At the outer edge, the resupply of water at low latitudes stabilizes the greenhouse effect and keeps the planet in the HD state and may prevent water from getting trapped at high latitudes in frozen form. Overall, the existence of bistability in the presence of an overland recycling mechanism hints at the possibility of a wider habitable zone for Earth-like terra-planets at low obliquities.
Highlights
Recent advancements in observational astrophysics like with the Kepler mission led to the discovery of a vast number of potentially habitable planets (Kopparapu et al, 2014)
Starting from the hot and dry (HD) state and increasing background soil albedo above the threshold causes an abrupt shift from the HD state to the cold and wet (CW) state resulting in a sudden cooling of about 35 ◦C globally, which is of the order of the temperature difference between present day and the Snowball Earth state
The reason for the absence of a CW state in previous studies is the lack of an effective mechanism that can recycle the water trapped at the high latitudes in the form of snow and ice back to the low latitudes
Summary
Recent advancements in observational astrophysics like with the Kepler mission led to the discovery of a vast number of potentially habitable planets (Kopparapu et al, 2014). The reason is that limited atmospheric access to water results in a dry climate with water confined to the high latitudes for low obliquities (Abe et al, 2005; Abe et al, 2011; Leconte et al, 2013) In such dry climates, the water vapour feedback is severely muted and the greenhouse warming is substantially lowered, which allows dry planets to maintain habitability even at higher stellar fluxes (Zsom et al, 2013). With our study we demonstrate that under certain conditions Earth-like terra-planets exhibit two drastically different climate states and both these climate states can support habitable areas with long-lived surface liquid water.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
