CO2 levels to maintain Gliese 581g to be habitable

Whether Gliese 581g (Gl 581g) is a truly existing exoplanet requires further confirmation. If it is, Gl 581g is the most likely habitable exoplanet discovered in so far because it sits squarely in the habitable zone of its parent star, M-dwarf Gliese 581. For incident stellar flux of 866 W m-2 and 0.2 surface albedo, the temperature at the substellar point is 332 K in the absence of atmospheric greenhouse effect. It implies that there must be a portion of area around the stellar point where surface temperature is above the freezing point of water (273.15 K), and thus the area is habitable. However, the exoplanet might have undergone runaway freezing when ice-albedo feedback is considered and if atmospheric greenhouse effect is not sufficiently strong. Using a fully coupled atmosphere-ocean general circulation model (AOGCM), we perform simulations with various CO2 levels, assuming that the planet is covered by an ocean with 4 km in depth. All other parameters used are from observational results for Gl 581g. Our simulations show that Gl 581g is almost completely covered by snow and ice with the present CO2 level of the Earth atmosphere, and sufficiently high CO2 levels are required to maintain Gl 581g with a stable open-water area, i.e., the Eyeball state. We will also show simulation results of atmospheric flow structures, dynamic sea-ice movement, and global hydrological cycle.