# Climate Sensitivity to Carbon Dioxide and Moist Greenhouse threshold of Earth-like planets under an increasing solar forcing

## TL;DR

This study uses a global climate model to identify the moist greenhouse threshold of Earth-like planets under increasing solar forcing, considering CO2 and ozone effects, with implications for planetary habitability.

## Contribution

It introduces the first analysis of the moist greenhouse threshold including ozone effects using an intermediate complexity GCM, linking it to climate sensitivity and water vapor inflection points.

## Key findings

- Moist greenhouse threshold occurs at 320 K surface temperature for CO2 between 560 ppm and 200 ppm.
- Threshold correlates with water vapor inflection point and peak in climate sensitivity.
- Results are consistent with complex GCMs, indicating robustness of the indicators.

## 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 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.