  The conventional observables to identify a habitable or inhabited environment
in exoplanets, such as an ocean glint or abundant atmospheric O$_2$, will be
challenging to detect with present or upcoming observatories. Here we suggest a
new signature. A low carbon abundance in the atmosphere of a temperate rocky
planet, relative to other planets of the same system, traces the presence of
substantial amount of liquid water, plate tectonic and/or biomass. We show that
JWST can already perform such a search in some selected systems like TRAPPIST-1
via the CO$_2$ band at $4.3\,\rm \mu m$, which falls in a spectral sweet spot
where the overall noise budget and the effect of cloud/hazes are optimal. We
propose a 3-step strategy for transiting exoplanets: 1) detection of an
atmosphere around temperate terrestrial planets in $\sim 10$ transits for the
most favorable systems, (2) assessment of atmospheric carbon depletion in $\sim
40$ transits, (3) measurements of O$_3$ abundance to disentangle between a
water- vs biomass-supported carbon depletion in $\sim100$ transits. The concept
of carbon depletion as a signature for habitability is also applicable for
next-generation direct imaging telescopes.
