A novel metric for assessing climatological surface habitability

TL;DR

This paper introduces a new climate habitability metric based on Earth's observed life limits, improving the assessment of potential life-supporting conditions on exoplanets beyond traditional temperature-based measures.

Contribution

It develops and validates a novel habitability metric incorporating thermal and water flux limits, expanding the scope beyond binary habitability definitions.

Findings

Temperature alone does not fully capture habitability patterns.

The new metric better aligns with observed Earth's habitability distribution.

Incorporating water fluxes improves habitability assessment accuracy.

Abstract

Planetary surface habitability has so far been considered, in the main, upon a global scale. The increasing number of 3D modelling studies of (exo)planetary climate has highlighted the need for a more nuanced understanding of surface habitability. Using satellite-derived data of photosynthetic life to represent the observed surface habitability of modern Earth, we validate a set of climatologically-defined metrics previously used in habitability studies. The comparison finds that the metrics defined by surface temperature alone show spatial patterns of habitability distinct to those defined by aridity or water availability, with no metric able to completely replicate the observed habitability. We build upon these results to introduce a new metric defined by the observed thermal limits of modern Earth-based life, along with surface water fluxes as an analogue for water and nutrient availability. Furthermore, we pay attention to not only the thermal bounds of macroscopic complex life, but additionally the limits of microbial life which have been vital to the generation of Earth's biosignatures, thus expanding considerations of climatic habitability out of a historically binary definition. Repeating the validation for our metric, along with another which uses a similar definition that incorporates conditions for both temperature and water availability, shows a significant improvement in capturing the broad patterns of surface habitability, laying the groundwork for more comprehensive assessments of potential life-supporting climates upon other planets.