An accurate estimate of the inner edge of the habitable zone is critical for determining which exoplanets are potentially habitable and for designing future telescopes to observe them. Here, we explore differences in estimating the inner edge among seven one-dimensional radiative transfer models: two line-by-line codes (SMART and LBLRTM) as well as five band codes (CAM3, CAM4_Wolf, LMDG, SBDART, and AM2) that are currently being used in global climate models. We compare radiative fluxes and spectra in clear-sky conditions around G and M stars, with fixed moist adiabatic profiles for surface temperatures from 250 to 360 K. We find that divergences among the models arise mainly from large uncertainties in water vapor absorption in the window region (10 μm) and in the region between 0.2 and 1.5 μm. Differences in outgoing longwave radiation increase with surface temperature and reach 10-20 W m −2 ; differences in shortwave reach up to 60 W m −2 , especially at the surface and in the troposphere, and are larger for an M-dwarf spectrum than a solar spectrum. Differences between the two lineby-line models are significant, although smaller than among the band models. Our results imply that the uncertainty in estimating the insolation threshold of the inner edge (the runaway greenhouse limit) due only to clear-sky radiative transfer is ≈10% of modern Earth's solar constant (i.e., ≈34 W m −2 in global mean) among band models and ≈3% between the two line-by-line models. These comparisons show that future work is needed that focuses on improving water vapor absorption coefficients in both shortwave and longwave, as well as on increasing the resolution of stellar spectra in broadband models.

Abstract.One of the critical issues of the Snowball Earth hypothesis is the CO 2 threshold for triggering the deglaciation. Using Community Atmospheric Model version 3.0 (CAM3), we study the problem for the CO 2 threshold. Our simulations show large differences from previous results (e.g. Pierrehumbert, 2004Pierrehumbert, , 2005Le Hir et al., 2007). At 0.2 bars of CO 2 , the January maximum near-surface temperature is about 268 K, about 13 K higher than that in Pierrehumbert (2004Pierrehumbert ( , 2005, but lower than the value of 270 K for 0.1 bar of CO 2 in Le Hir et al. (2007). It is found that the difference of simulation results is mainly due to model sensitivity of greenhouse effect and longwave cloud forcing to increasing CO 2 . At 0.2 bars of CO 2 , CAM3 yields 117 Wm −2 of clear-sky greenhouse effect and 32 Wm −2 of longwave cloud forcing, versus only about 77 Wm −2 and 10.5 Wm −2 in Pierrehumbert (2004, 2005), respectively. CAM3 has comparable clear-sky greenhouse effect to that in Le Hir et al. (2007), but lower longwave cloud forcing. CAM3 also produces much stronger Hadley cells than that in Pierrehumbert (2005).Effects of pressure broadening and collision-induced absorption are also studied using a radiative-convective model and CAM3. Both effects substantially increase surface temperature and thus lower the CO 2 threshold. The radiativeconvective model yields a CO 2 threshold of about 0.21 bars with surface albedo of 0.663. Without considering the effects of pressure broadening and collision-induced absorption, CAM3 yields an approximate CO 2 threshold of about 1.0 bar for surface albedo of about 0.6. However, the threshold is lowered to 0.38 bars as both effects are considered.

Aims. The M-type star Gliese 581 is likely to have two super-Earth planets, i.e., Gl 581c and Gl 581d. The present study is to investigate their habitability constrained by radiative properties of their atmospheres and the threshold of carbon-dioxide (CO 2 ), assuming that the two exoplanets are terrestrial, and that they have similar outgassing processes to those of the terrestrial planets in our own solar system. Methods. Radiative-convective atmospheric models are used. Different values of CO 2 concentrations and water-vapor mixing ratios are tested. Results. Our simulation results suggest that Gl 581d is probably a habitable planet. However, at least 6.7 bars of CO 2 are required to raise its surface temperature (T s ) above the freezing point of water. In contrast, Gl 581c might have experienced runaway greenhouse, like Venus, because of its too high surface temperature and the lack of an effective cold trap for water vapor. We compare our results with other independent studies.

Condensible substances are nearly ubiquitous in planetary atmospheres. For the most familiar case-water vapor in Earth's present climate-the condensible gas is dilute, in the sense that its concentration is everywhere small relative to the noncondensible background gases. A wide variety of important planetary climate problems involve nondilute condensible substances. These include planets near or undergoing a water vapor runaway and planets near the outer edge of the conventional habitable zone, for which CO 2 is thecondensible. Standard representations of convection in climate models rely on several approximations appropriate only to the dilute limit, while nondilute convection differs in fundamental ways from dilute convection. In this paper, a simple parameterization of convection valid in the nondilute as well as dilute limits is derivedand used to discuss the basic character of nondilute convection. The energy conservation properties of the scheme are discussed in detailand are verified in radiative-convective simulations. As a further illustration of the behavior of the scheme, results for a runaway greenhouse atmosphere forbothsteady instellation and seasonally varying instellation corresponding to a highly eccentric orbit are presented. The latter case illustrates that the high thermal inertia associated with latent heat in nondilute atmospheres can damp out the effects of even extreme seasonal forcing.

Most previous studies on how obliquity affects planetary habitability focused on planets around Sun-like stars. Their conclusions may not be applicable to habitable planets around M dwarfs due to the tidal-locking feature and associated insolation pattern of these planets. Here we use a comprehensive three-dimensional atmospheric general circulation model to investigate this issue. We find that the climates of planets with higher obliquities are generally warmer, consistent with previous studies. The mechanism of warming is, however, completely different. Significant reduction of low clouds, instead of sea-ice cover, within the substeller region (which moves if the obliquity is non-zero) is the key in warming M-dwarf planets with high obliquities. For a total insolation of 1237 W m −2 , the climate warms by 21 K when the obliquity increases from 0°to 90°. Correspondingly, the runaway greenhouse inner edge of the habitable zone shifts outward from 2500 to 2100 W m −2 . The moist greenhouse inner edge, based on our crude estimation, shifts less, from 2180 to 2075 W m −2 . Near the outer edge, in contrast, the climates of planets with higher obliquities are colder due to their reduced ability to maintain a hotspot at the surface. Therefore, the outer edge moves inward when obliquity is increased, opposite to the finding of previous studies on planets around Sun-like stars. Our results thus indicate that the habitable zone for M dwarfs narrows if the obliquity of their planets increases.