<|endoftext|> However, this work was not done with a 3-D GCM and therefore lacked a full treatment of clouds and atmosphere-ocean interactions. Using a 3-D GCM, we find that the only scenario that permits Kepler-62f to exhibit clement temperatures for surface liquid water with 3 bar of CO 2 in the planet's atmosphere requires stringent orbital configuration requirements (high eccentricity and obliquity). With an obliquity of 90°, open water was present over just ∼20% of the planet, at polar latitudes. This indicates that it is likely that more than 1.6 bar of CO 2 is required for surface habitability on this planet.
We assumed a five-planet system for Kepler-62 in our n-body simulations. While there is currently no observational evidence for additional planets, the existence of other planets in this system is a clear possibility, and their presence would certainly affect the eccentricity limit for dynamical stability that we have calculated here. Future discovery of additional planets in this system with transit timing variations or RV may provide additional dynamical constraints on the eccentricity of Kepler-62f.
We combined climate simulations using CCSM4 and LMD Generic GCM to provide a comprehensive exploration of the possible climates for Kepler-62f given its n-body model constraints. Previous work using LMD Generic GCM at a higher resolution (e.g., 128 × 96) yielded similar results to those at the resolution we employed here (64 × 48), and as we confirmed that the global mean surface temperature had not changed by more than 1 K in the last 20 years of simulation, we are confident that running our simulations for longer timescales would not change our results significantly. We verified that simulations assuming Earth-like CO 2 levels in both GCMs exhibited similar (ice-covered) conditions, confirming the robustness of these simulations to various assumptions.
Our simulations with higher levels of CO 2 resulted in increasingly higher surface temperatures on the planet. As CO 2 was increased, the changes in global mean surface temperatures became progressively smaller. This logarithmic relationship between CO 2 concentration and radiative forcing is long established (Wigley, 1987). As regions of the spectrum become opaque, additional CO 2 molecules become far less effective at increasing temperatures (Shine et al., 1990). Thus, the greenhouse effect starts to become less efficient as a warming mechanism. Additionally, CO 2 is 2.5× more effective as a Rayleigh scatterer than Earth's air (Kasting, 1991; Forget and Pierrehumbert, 1997), and this behavior likely contributes to the loss of warming at higher CO 2 concentrations (Kasting, 1991; Kasting et al., 1993; Selsis et al., 2007).
We have not included the effect of CO 2 condensation in our simulations. As discussed in Section 2.4, at levels of 1–2 bar, CO 2 condensation is likely to occur in the upper atmosphere (Pierrehumbert, 2005). Depending on the particle size of CO 2 ice grains, this could result in cooling of the planet due to the albedo effect of CO 2 ice clouds (Kasting, 1991) or warming by scattering outgoing thermal radiation back toward the surface of the planet (Forget and Pierrehumbert, 1997).
Kopparapu et al. (2013a, 2013b) found that the maximum CO 2 greenhouse limit is ∼7–8 bar for a star with a similar effective temperature to that of Kepler-62 (Kopparapu et al., 2013a, 2013b). We did find that the planetary albedo, which had decreased with increasing CO 2 concentration, started to increase, albeit slowly, at a CO 2 level of 8 bar. However, our simulations with 8–12 bar of CO 2 resulted in global mean surface temperatures that were still higher than those with lower CO 2 , so we conclude that we had not yet reached the maximum CO 2 limit in our simulations and that it may be higher than originally proposed. Kopparapu et al. (2013a, 2013b) used a 1-D radiative-convective model in their work and did not include the effect of water clouds or CO 2 clouds in their calculations. While water clouds could increase the planetary albedo, thereby cooling the planet further, they may also contribute to the greenhouse effect, as both H 2 O and CO 2 have strong absorption coefficients in the near-IR, which increase the amount of radiation absorbed by planets with lower-mass host stars that emit strongly in the near-IR (Kasting et al., 1993; Selsis et al., 2007; Joshi and Haberle, 2012; Kopparapu et al., 2013a, 2013b; Shields et al., 2013, 2014). Our results with a 3-D GCM do include water clouds, though not CO 2 clouds. A comprehensive study of the effect of CO 2 condensation as