Abstract
Abstract Stable, steady climate states on an Earth-size planet with no continents are determined as a function of the tilt of the planet’s rotation axis (obliquity) and stellar irradiance. Using a general circulation model of the atmosphere coupled to a slab ocean and a thermodynamic sea ice model, two states, the Aquaplanet and the Cryoplanet, are found for high and low stellar irradiance, respectively. In addition, four stable states with seasonally and perennially open water are discovered if comprehensively exploring a parameter space of obliquity from 0° to 90° and stellar irradiance from 70% to 135% of the present-day solar constant. Within 11% of today’s solar irradiance, we find a rich structure of stable states that extends the area of habitability considerably. For the same set of parameters, different stable states result if simulations are initialized from an aquaplanet or a cryoplanet state. This demonstrates the possibility of multiple equilibria, hysteresis, and potentially rapid climate change in response to small changes in the orbital parameters. The dynamics of the atmosphere of an aquaplanet or a cryoplanet state is investigated for similar values of obliquity and stellar irradiance. The atmospheric circulation substantially differs in the two states owing to the relative strength of the primary drivers of the meridional transport of heat and momentum. At 90° obliquity and present-day solar constant, the atmospheric dynamics of an Aquaplanet state and one with an equatorial ice cover is analyzed.
Highlights
Habitability on exoplanets for known forms of life depends on the presence of liquid water
The Planet Simulator, PlaSim, is used (Lunkeit et al 2011). It is a climate model of intermediate complexity consisting of a general circulation model for the atmosphere (AGCM) coupled to a slab ocean and a thermodynamic sea ice model
Potential effects of changing ocean circulation cannot be captured with our model. This could have important consequences for the meridional distribution of heat and the presence of multiple equilibria, recent simulations suggest that the overall effect of ocean dynamics is limited and that a slab ocean is a good approximation for the major feedback processes involving the storage and distribution of heat by the ocean (Ferreira et al 2014)
Summary
Habitability on exoplanets for known forms of life depends on the presence of liquid water. Going beyond earlier studies that used highly simplified energy balance models to illustrate possible climate states of exoplanets (Spiegel et al 2009), three-dimensional atmospheric models (Shields et al 2013, 2014), and even comprehensive climate models for a small number of parameter values, such as stellar irradiation or obliquities (Ferreira et al 2014; Hu & Yang 2014), here we employ a computationally efficient climate model of intermediate complexity, the Planet Simulator (PlaSim) developed by Lunkeit et al (2011).
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