  Robustly modeling the inner edge of the habitable zone is essential for
determining the most promising potentially habitable exoplanets for atmospheric
characterization. Global climate models (GCMs) have become the standard tool
for calculating this boundary, but divergent results have emerged among the
various GCMs. In this study, we perform an inter-comparison of standard GCMs
used in the field on a rapidly rotating planet receiving a G-star spectral
energy distribution and on a tidally locked planet receiving an M-star spectral
energy distribution. Experiments both with and without clouds are examined. We
find relatively small difference (within 8 K) in global-mean surface
temperature simulation among the models in the G-star case with clouds. In
contrast, the global-mean surface temperature simulation in the M-star case is
highly divergent (20-30 K). Moreover, even differences in the simulated surface
temperature when clouds are turned off are significant. These differences are
caused by differences in cloud simulation and/or radiative transfer, as well as
complex interactions between atmospheric dynamics and these two processes. For
example we find that an increase in atmospheric absorption of shortwave
radiation can lead to higher relative humidity at high altitudes globally and,
therefore, a significant decrease in planetary radiation emitted to space. This
study emphasizes the importance of basing conclusions about planetary climate
on simulations from a variety of GCMs and motivates the eventual comparison of
GCM results with terrestrial exoplanet observations to improve their
performance.
