  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.
