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An accurate estimate of the inner edge of the habitable zone is critical fordetermining which exoplanets are potentially habitable and for designing futuretelescopes to observe them. Here, we explore differences in estimating theinner edge among seven one-dimensional (1D) radiative transfer models: twoline-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 globalclimate models. We compare radiative fluxes and spectra in clear-sky conditionsaround G- and M-stars, with fixed moist adiabatic profiles for surfacetemperatures from 250 to 360 K. We find that divergences among the models arisemainly from large uncertainties in water vapor absorption in the window region(10 um) and in the region between 0.2 and 1.5 um. Differences in outgoinglongwave radiation increase with surface temperature and reach 10-20 Wm^-2;differences in shortwave reach up to 60 Wm^-2, especially at the surface and inthe troposphere, and are larger for an M-dwarf spectrum than a solar spectrum.Differences between the two line-by-line models are significant, althoughsmaller than among the band models. Our results imply that the uncertainty inestimating the insolation threshold of the inner edge (the runaway greenhouselimit) due only to clear-sky radiative transfer is ~10% of modern Earth's solarconstant (i.e., ~34 Wm^-2 in global mean) among band models and ~3% between thetwo line-by-line models. These comparisons show that future work is neededfocusing on improving water vapor absorption coefficients in both shortwave andlongwave, as well as on increasing the resolution of stellar spectra inbroadband models.

Differences in water vapor radiative transfer among 1D models can significantly affect the inner edge of the habitable zone