Abstract
We explore the potential multistability of the climate for a planet around
the habitable zone. We focus on conditions reminiscent to those of the Earth
system, but our investigation aims at presenting a general methodology for
dealing with exoplanets. We provide a thorough analysis of the non-equilibrium
thermodynamical properties of the climate system and explore, using a a
flexible climate model, how such properties depend on the energy input of the
parent star, on the infrared atmospheric opacity, and on the rotation rate. It
is possible to reproduce the multi-stability properties reminiscent of the
paleoclimatologically relevant snowball (SB) - warm (W) conditions. We then
study the thermodynamics of the W and SB states, clarifying the role of the
hydrological cycle in shaping the irreversibility and the efficiency of the W
states, and emphasizing the extreme diversity of the SB states, where dry
conditions are realized. Thermodynamics provides the clue for studying the
tipping points of the system and leads us to constructing parametrizations
where the main thermodynamic properties are expressed as functions of the
emission temperature of the planet only. Such functions are rather robust with
respect to changing the rotation rate of the planet from the current
terrestrial one to half of it. We then explore the dynamical range of slowy
rotating and phase locked planets. There is a critical rotation rate below
which the multi-stability properties are lost. Such critical rotation rate
corresponds roughly to the phase lock 2:1 condition. Therefore, if an
Earth-like planet is 1:1 phase locked with respect to the parent star, only one
climatic state would be compatible with a given set of astronomical and
astrophysical parameters. These results have relevance for the general theory
of planetary circulation and for the definition of necessary and sufficient
conditions for habitability.
