Dependence of the Runaway Threshold on Surface Water Distributions of Earth-like Planets

Abstract:

Liquid water is one of the most important material not only for its large effect on planetary climate but also as a controlling factor of the habitability [e.g. Kasting et al., 1993]. Liquid water on the surface of a planet gets unstable and entirely vaporizes when the planet receives insolation above a certain critical value. In the following, the critical insolation is called the ‘runaway threshold’ [e.g. Kasting, 1988; Nakajima et al., 1992].

Recently, it is found that the runaway threshold strongly depends on the general circulation of the planetary atmosphere [Abe et al., 2011; Leconte et al., 2013; Wolf and Toon, 2014].

The planetary-scale surface water distribution is determined by the balance between the atmospheric water transport, which depends on the general circulation, and the surface water transport, which depends on the amount of water and topography. In the case of the Earth, large amount of sea water controls the surface water distribution.

Generally, the atmospheric circulation transports water poleward, and in contrast, the surface waterflow homogenizes the distribution of surface water. When the surface waterflow is weak, surface water is localized around both poles. On the other hand, if the surface waterflow is strong, liquid water is found at any latitude. We call the former planet a ‘land planet’, and the latter planet an ‘aqua planet’.

Abe et al. [2011] discussed the difference of the runaway threshold between Earth-sized aqua and land planets using a 3-D model for the first time. They found that the surface water on a land planet is significantly more stable than that on an aqua planet against the large insolation. However, they discussed only 2 extreme cases of land and aqua planets.

Here, we report results of numerical experiments to clarify the effect of waterflow on the runaway threshold of Earth-sized water planets with CCSR/NIES AGCM 5.4g [Numaguchi, 1999]. We assume the present Earth atmosphere with several simplifications.

The strength of waterflow is expressed by the `waterflow limit` above which latitude the surface waterflow keeps a wet surface. Then, we found that the runaway threshold continuously varies with the waterflow limit from 180% of the present insolation at the Earth orbit (the case of extremely localized land planet), to 130% (aqua planet).