Climate Diversity in the Solar-Like Habitable Zone due to Varying Background Gas Pressure

TL;DR

This study uses a 3D climate model to explore how varying background gas pressure, especially N$_2$, influences the climate and observational signatures of Earth-like exoplanets in the habitable zone, revealing nonlinear sensitivities and key mechanisms.

Contribution

It is the first comprehensive analysis of how background gas partial pressure, particularly N$_2$, affects climate states and observational signals of terrestrial exoplanets.

Findings

Climate response to N$_2$ pressure is nonlinear and sensitive.

Pressure broadening and water vapor feedback are key mechanisms.

N$_2$ levels significantly impact exoplanet reflected light observations.

Abstract

A large number of studies have responded to the growing body of confirmed terrestrial habitable zone exoplanets by presenting models of various possible climates. However, the impact of the partial pressure of background gases such as N$_2$ has not yet been well-explored, despite the abundance of N$_2$ in Earth's atmosphere and the lack of constraints on its typical abundance in terrestrial planet atmospheres. We use PlaSim, a fast 3D climate model, to simulate many hundreds of climates around Sun-like stars with varying N$_2$ partial pressures, instellations, and surface characteristics to identify the impact of the background gas partial pressure on the climate. We find that the climate's response is nonlinear and highly sensitive to the background gas partial pressure. We identify pressure broadening of greenhouse gas (such as CO$_2$ and H$_2$O) absorption lines, amplification of warming or cooling by the water vapor greenhouse positive feedback, heat transport efficiency, and cooling through Rayleigh scattering as the dominant competing mechanisms that determine the equilibrium climate for a given N$_2$ partial pressure. Finally, we show that different amounts of N$_2$ should have a significant effect on broadband reflected light observations of terrestrial exoplanets around Sun-like stars.