A novel metric for assessing climatological surface habitability

Abstract: Planetary surface habitability has so far been, in the main, considered in its entirety. The increasing popularity 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 exoplanetary habitability studies. The comparison finds that the metrics defined by temperature show spatial patterns of habitability distinct to those defined by aridity, with no metric able to completely replicate the observed. 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 own biosignatures, thus expanding considerations of climatic habitability out of a historically binary definition. Repeating the validation for our metric finds a significant improvement in the spatial representation of habitability, laying the groundwork for more accurate assessments of potential life-supporting environments upon other planets.