Climate Sensitivity to Carbon Dioxide and the Moist Greenhouse Threshold of Earth-like Planets under an Increasing Solar Forcing

Abstract

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 CO 2 concentrations and including the radiative effect of atmospheric ozone for the first time. We show that the MGT correlates with the inflection point in the water vapor mixing ratio in the stratosphere and a peak in the climate sensitivity. For CO 2 concentrations between 560 and 200 ppm, the MGT 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 the inner edge of the habitable zone as well as the water loss timescales for Earth analog planets.