  We examine the fine structure lines $^{3}P_{1}\to $$^{3}P_{0}$ (492 GHz) and
$ ^{3}P_2\to $$^{3}P_1$ (809 GHz) of neutral atomic carbon as bulk molecular
gas mass tracers and find that they can be good and on many occasions better
than $ ^{12}$CO transitions, especially at high redshifts. The notion of CI
emission as an H$_2$ gas mass tracer challenges the long-held view of its
distribution over only a relatively narrow layer in the CII/CI/CO transition
zone in FUV-illuminated molecular clouds. Past observations have indeed
consistently pointed towards a more extended CI distribution but it was only
recently, with the advent of large scale imaging of its $^{3}P_{1}\to
$$^{3}P_{0}$ transition, that its surprising ubiquity in molecular clouds has
been fully revealed. In the present work we show that under {\it typical} ISM
conditions such an ubiquity is inevitable because of well known dynamic and
non-equilibrium chemistry processes maintaining a significant [C]/[$ ^{12}$CO]
abundance throughout Giant Molecular Clouds during their lifetime. These
processes are more intense in star-forming environments where a larger ambient
cosmic ray flux will also play an important role in boosting [C]/[$ ^{12}$CO].
The resulting CI lines can be bright and effective H$_2$ mass tracers
especially for diffuse ($\sim 10^2-10^3\rm cm^{-3}$) gas while in UV-intense
and/or metal-poor environments their H$_2$-tracing capability diminishes
because of large scale CII production but nevertheless remains superior to that
of $ ^{12}$CO. The best place to take full advantage of CI's capacity to trace
H$_2$ is not in the low-$z$ Universe, where large atmospheric absorption at 492
and 809 GHz precludes routine observations, but at high redshifts ($\rm z\ga
1$).
