Stratospheric Temperatures and Water Loss from Moist Greenhouse Atmospheres of Earth-like Planets

TL;DR

This study uses a climate model to analyze how increasing surface temperatures affect stratospheric water vapor and temperature, highlighting implications for water loss and habitability of Earth-like exoplanets.

Contribution

It demonstrates that at high surface temperatures, the stratosphere warms significantly, enabling water loss, and emphasizes the importance of accurate temperature modeling for habitable zone assessments.

Findings

Stratospheres are cold and dry at low temperatures.

Above 350 K, the stratosphere warms and water vapor increases.

Upper atmospheric temperatures are lower than gray atmosphere estimates.

Abstract

A radiative-convective climate model is used to calculate stratospheric temperatures and water vapor concentrations for ozone-free atmospheres warmer than that of modern Earth. Cold, dry stratospheres are predicted at low surface temperatures, in agreement with recent 3-D calculations. However, at surface temperatures above 350 K, the stratosphere warms and water vapor becomes a major upper atmospheric constituent, allowing water to be lost by photodissociation and hydrogen escape. Hence, a 'moist greenhouse' explanation for loss of water from Venus, or some exoplanet receiving a comparable amount of stellar radiation, remains a viable hypothesis. Temperatures in the upper parts of such atmospheres are well below those estimated for a gray atmosphere, and this factor should be taken into account when performing 'inverse' climate calculations to determine habitable zone boundaries using 1-D models.