Synergistic Effects of Ocean Salinity and Planetary Obliquity Enhance Habitability of Cold Exo-Earths

TL;DR

This study demonstrates that ocean salinity and planetary obliquity interact synergistically to significantly influence the climate and habitability of cold exo-Earths, with implications for exoplanet characterization.

Contribution

It is the first to explore the combined effects of salinity and obliquity on exoplanet climate using coupled climate models, revealing their joint impact on habitability.

Findings

Salinity and obliquity together have a greater climate impact than individually.

The synergy affects climate states from ice-free to globally glaciated.

Habitability can be increased by their combined effects, especially for cold exoplanets.

Abstract

Past work has shown that ocean salinity and planetary obliquity both influence the climates of Earth-like exoplanets throughout the habitable zone of Sun-like stars. The effects of salinity and obliquity can be profound, with low vs. high salinity or obliquity resulting in distinct climate states in some scenarios. However, past work has considered salinity or obliquity in isolation and has not explored how each may modulate the effects of the other. We investigate how ocean salinity and planetary obliquity jointly impact climate and habitability using the ROCKE-3D coupled ocean-atmosphere general circulation model. We find that salinity and obliquity have a greater combined impact on planetary climate than the sum of their effects in isolation. This synergy between salinity and obliquity arises due to the ice-albedo feedback, producing distinct climate states that range from ice-free to globally glaciated while having same initial atmospheric conditions and receiving the same instellation. Consequently, ocean salinity and planetary obliquity can together lead to divergent habitability outcomes for otherwise identical planetary scenarios and initial conditions. Salinity and obliquity can jointly increase the planetary fractional habitability across oceans and continents, especially for cold exoplanets. Although neither ocean salinity nor planetary obliquity can be reliably predicted or observationally constrained, their synergistic effects must be considered in future studies of planetary climate and exoplanet observations, especially when characterizing planetary habitability.