How does the background atmosphere affect the onset of the runaway greenhouse?

Abstract

As the insolation of an Earth-like (exo)planet with a large amount of water increases, its surface and atmospheric temperatures also increase, eventually leading to a catastrophic runaway greenhouse transition. While some studies have shown that the onset of the runaway greenhouse may be delayed due to an overshoot of the outgoing longwave radiation (OLR) – compared to the Simpson-Nakajima threshold – by radiatively inactive gases, there is still no consensus on whether this is occurring and why. Here, we used a suite of 1D radiative-convective models to study the runaway greenhouse transition, with particular emphasis on taking into account the radical change in the amount of water vapour (from trace gas to dominant gas). The aim of this work is twofold: first, to determine the most important physical processes and parametrisations affecting the OLR; and second, to propose reference OLR curves for N2+H2O atmospheres. Through multiple sensitivity tests, we list and select the main important physical processes and parametrisations that need to be accounted for in 1D radiative-convective models to compute an accurate estimate of the OLR for N2+H2O atmospheres. The reference OLR curve is computed with a 1D model built according to the sensitivity tests. These tests also allow us to interpret the diversity of results already published in the literature. Moreover, we provide a correlated-k table able to reproduce line-by-line calculations with high accuracy. We find that the transition between an N2-dominated atmosphere and an H2O-dominated atmosphere induces an overshoot of the OLR compared to the (pure H2O) Simpson–Nakajima asymptotic limit. This overshoot is first due to a transition between foreign and self-broadening of the water absorption lines, and second to a transition between dry and moist adiabatic lapse rates.

Early Formation of a Water Ocean as a Function of Initial CO2 and H2O Contents in a Solidifying Rocky Planet

“…The more massive atmosphere of those sub‐neptune worlds should lead to deviation on greenhouse limits (e.g., Pierrehumbert, 2023). Also, the atmospheres of smaller rocky exoplanets may contain other volatile species than H 2 O or CO 2 , leading to complex chemical reactions as a function of their abundance and temperature (e.g., Wordsworth & Kreidberg, 2022) thus influencing the inflection point of the runaway greenhouse (e.g., Boukrouche et al., 2021; Chaverot et al., 2022).…”

Early formation of a water ocean as a function of initial CO2 and H2O contents in a solidifying rocky planet

“…The more massive atmosphere of those sub-neptune worlds should lead to deviation on greenhouse limits (e.g., Pierrehumbert, 2023). Also, the atmospheres of smaller rocky exoplanets may contain other volatile species than H 2 O or CO 2 , leading to complex chemical reactions as a function of their abundance and temperature (e.g., Wordsworth & Kreidberg, 2022) thus influencing the inflection point of the runaway greenhouse (e.g., Boukrouche et al, 2021;Chaverot et al, 2022).…”

The Feasibility of Detecting Biosignatures in the TRAPPIST-1 Planetary System with JWST

“…Unlike Goldblatt et al (2013), our climate calculations are self-consistent and take into account a realistic atmospheric composition, which includes N 2 and CO 2 . These gases are known to modify the runaway greenhouse onset from that anticipated from a pure water vapor atmosphere (Chaverot et al 2022). We also use the M-dwarf host star's stellar spectrum, which produces a different planetary temperature structure than that used in Goldblatt et al (2013).…”