Dependence of the Onset of the Runaway Greenhouse Effect on the Latitudinal Surface Water Distribution of Earth‐Like Planets

• Published in: Journal of geophysical research. Planets 2018-02, Vol.123 (2), p.559-574
• Main Authors: Kodama, T., Nitta, A., Genda, H., Takao, Y., O'ishi, R., Abe‐Ouchi, A., Abe, Y.
• Format: Article
• Language: English
• Subjects: Atmosphere; atmospheric circulation; Atmospheric models; Cloud cover; Earth; Earth atmosphere; Earth orbits; Earth surface; Greenhouse effect; Habitability; habitable planets; Hadley circulation; Insolation; Latitude; Planets; runaway greenhouse effect; Surface water; Terrestrial planets; Venus; Water; Water circulation; Water distribution; Water engineering
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1002/2017JE005383

Liquid water is one of the most important materials affecting the climate and habitability of a terrestrial planet. Liquid water vaporizes entirely when planets receive insolation above a certain critical value, which is called the runaway greenhouse threshold. This threshold forms the inner most limit of the habitable zone. Here we investigate the effects of the distribution of surface water on the runaway greenhouse threshold for Earth‐sized planets using a three‐dimensional dynamic atmosphere model. We considered a 1 bar atmosphere whose composition is similar to the current Earth's atmosphere with a zonally uniform distribution of surface water. As previous studies have already showed, we also recognized two climate regimes: the land planet regime, which has dry low‐latitude and wet high‐latitude regions, and the aqua planet regime, which is globally wet. We showed that each regime is controlled by the width of the Hadley circulation, the amount of surface water, and the planetary topography. We found that the runaway greenhouse threshold varies continuously with the surface water distribution from about 130% (an aqua planet) to 180% (the extreme case of a land planet) of the present insolation at Earth's orbit. Our results indicate that the inner edge of the habitable zone is not a single sharp boundary, but a border whose location varies depending on planetary surface condition, such as the amount of surface water. Since land planets have wider habitable zones and less cloud cover, land planets would be good targets for future observations investigating planetary habitability. Key Points The onset of the runaway greenhouse effect depends strongly on the surface water distribution The runaway threshold increases as the surface water distribution retreats toward higher latitude outside the Hadley circulation The lower the water amount on a terrestrial planet, the longer the planet remains in habitable condition