How does the background atmosphere affect the onset of the runaway greenhouse? Insights from 1D radiative-convective modeling.

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