  Carbon dioxide is one of the major contributors to the radiative forcing,
increasing both the temperature and the humidity of Earth's atmosphere. If the
stellar irradiance increases and water becomes abundant in the stratosphere of
an Earth-like planet, it will be dissociated and the resultant hydrogen will
escape from the atmosphere. This state is called the moist greenhouse threshold
(MGT). Using a global climate model (GCM) of intermediate complexity, we
explore how to identify this state for different CO2 concentrations and
including the radiative effect of atmospheric ozone for the first time. We show
that the moist greenhouse threshold correlates with the inflection point in the
water vapor mixing ratio in the stratosphere and a peak in the climate
sensitivity. For CO2 concentrations between 560 ppm and 200 ppm, the moist
greenhouse threshold is reached at a surface temperature of 320 K. Despite the
higher simplicity of our model, our results are consistent with similar
simulations without ozone by complex GCMs, suggesting that they are robust
indicators of the MGT. We discuss the implications for inner edge of the
habitable zone as well as the water loss timescales for Earth analog planets.
