How does the background atmosphere affect the onset of the runaway greenhouse? Insights from 1D radiative-convective modeling.
Authors: Chaverot Guillaume , Bolmont Emeline , Turbet Martin , Leconte Jérémy 
Published: March 2021

&lt;p&gt;&lt;span&gt;There is a strong interest to study the runaway greenhouse effect &lt;/span&gt;&lt;span&gt;[1-4] &lt;/span&gt;&lt;span&gt;to better determ&lt;/span&gt;&lt;span&gt;ine&lt;/span&gt;&lt;span&gt; the runaway greenhouse insolation threshold &lt;/span&gt;&lt;span&gt;and therefore the inner edge of the habitable zone &lt;/span&gt;&lt;span&gt;(HZ)&lt;/span&gt;&lt;span&gt;. Some studies &lt;/span&gt;&lt;span&gt;[5-7] &lt;/span&gt;&lt;span&gt;have shown that the &lt;/span&gt;&lt;span&gt;onset of &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;runaway greenhouse may be delayed due to an increase of the Outgoing Longwave Radiation (OLR) by adding radiatively inactive gas (e.g. N&lt;sub&gt;2&lt;/sub&gt; or O&lt;sub&gt;2&lt;/sub&gt;, as in the Earth's atmosphere). &lt;/span&gt;&lt;span&gt;For such atmosphere t&lt;/span&gt;&lt;span&gt;he OLR may &amp;#8220;overshoot&amp;#8221; the &lt;/span&gt;Simpson-Nakajima limit [4], i.e. the moist greenhouse limit of a pure vapor atmopshere. &lt;span&gt;This has direct consequences on the position of the inner edge of the HZ [8-11] and thus on how clo&lt;/span&gt;&lt;span&gt;se the Earth is from a catastrophic runaway greenhouse feedback&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;The OLR overshoot has previously been interpreted as&lt;/span&gt;&lt;span&gt; a modification of the atmospheric profile due to the background gas &lt;/span&gt;&lt;span&gt;[7,12]&lt;/span&gt;&lt;span&gt;. However there is still no consensus so far in the literature on whether an OLR overshoot is &lt;/span&gt;&lt;span&gt;really &lt;/span&gt;&lt;span&gt;expected or not.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;The first aim of &lt;/span&gt;&lt;span&gt;our&lt;/span&gt;&lt;span&gt; work &lt;/span&gt;&lt;span&gt;is&lt;/span&gt;&lt;span&gt; to determine, through sensitivity tests, the main important physical processes and parametrizations involved in the OLR computation with a suite of 1D radiative-convective models. By doing multiple sensitivity experiments we are able to explain the origin of the differences in the results of the literature for a H&lt;sub&gt;2&lt;/sub&gt;O+N&lt;sub&gt;2&lt;/sub&gt; atmosphere. We showed that physical processes usually assumed as second order effects are actually key to explain the shape of the OLR &lt;/span&gt;(e.g., line shape parameters)&lt;span&gt;. This work can also be useful to &lt;/span&gt;&lt;span&gt;guide future&lt;/span&gt;&lt;span&gt; 3D GCM simulations. &lt;/span&gt;&lt;span&gt;We propose also preliminary results from the LMD-Generic model to &lt;/span&gt;&lt;span&gt;study&lt;/span&gt;&lt;span&gt; how these effects may be &lt;/span&gt;&lt;span&gt;understand&lt;/span&gt;&lt;span&gt; in a 3D simulation.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Secondly we propose a reference OLR curve, done with a &lt;/span&gt;&lt;span&gt;1D &lt;/span&gt;&lt;span&gt;model built according to the sensitivity &lt;/span&gt;&lt;span&gt;tests&lt;/span&gt;&lt;span&gt;, for a H&lt;sub&gt;2&lt;/sub&gt;O+N&lt;sub&gt;2&lt;/sub&gt; atmosphere, to solve the question of the potential overshoot. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;[1] Komabayasi, M. 1967, Journal of the Meteorological Society of Japan. Ser. II&lt;/p&gt;&lt;p&gt;[2] Ingersoll, A. 1969&lt;/p&gt;&lt;p&gt;[3] Nakajima, S., Hayashi, Y.-Y., &amp; Abe, Y. 1992, Journal of the Atmospheric Sci&lt;span&gt;ences&lt;/span&gt;&lt;/p&gt;&lt;p&gt;[4] Goldblatt, C. &amp; Watson, A. J. 2012, Philosophical Transactions of the Royal &lt;span&gt;Society A: Mathematical, Physical and Engineering Sciences&lt;/span&gt;&lt;/p&gt;&lt;p&gt;[5] Goldblatt, C., Claire, M. W., Lenton, T. M., et al. 2009, Nature Geoscience&lt;/p&gt;&lt;p&gt;[6] Goldblatt, C., Robinson, T. D., Zahnle, K. J. et al., 2013, Nature Geo&lt;span&gt;science&lt;/span&gt;&lt;/p&gt;&lt;p&gt;[7] Koll, D. D. B. &amp; Cronin, T. W. 2019, The Astrophysical Journal&lt;/p&gt;&lt;p&gt;[8] Leconte, J., Forget, F., Charnay, B. et al., 2013, Nature&lt;/p&gt;&lt;p&gt;[9] Kopparapu, R. k., Ramirez, R., Kasting, J. F., et al. 2013, The Astrophysical &lt;span&gt;Journal&lt;/span&gt;&lt;/p&gt;&lt;p&gt;[10] Ramirez, R. M. 2020, Monthly Notices of the Royal Astronomical Society&lt;/p&gt;&lt;p&gt;[11] Zhang, Y. &amp; Yang, J. 2020, The Astrophysical Journal&lt;/p&gt;&lt;p&gt;[12] Pierrehumbert, R. T. 2010, Principles of planetary climate&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;