  Evolution of the cosmic star formation rate (SFR) and molecular gas mass
density is expected to be matched by a similarly strong evolution of the
fraction of atomic hydrogen (HI) in the cold neutral medium (CNM). We use
results from a recent commissioning survey for intervening 21-cm absorbers with
the Australian Square Kilometre Array Pathfinder (ASKAP) to construct a
Bayesian statistical model of the $N_{\rm HI}$-weighted harmonic mean spin
temperature ($T_{\rm s}$) at redshifts between $z = 0.37$ and $1.0$. We find
that $T_{\rm s} \leq 274$K with 95 per cent probability, suggesting that at
these redshifts the typical HI gas in galaxies at equivalent DLA column
densities may be colder than the Milky Way interstellar medium ($T_{\rm s, MW}
\sim 300$K). This result is consistent with an evolving CNM fraction that
mirrors the molecular gas towards the SFR peak at $z \sim 2$. We expect that
future surveys for HI 21-cm absorption with the current SKA pathfinder
telescopes will provide constraints on the CNM fraction that are an order of
magnitude greater than presented here.
