Molecular gas mass density derived by SPRITZ from [C II], using the [C II] luminosity derived from V15 and the relation by Madden et al. (2020, dash-dotted orange line). We also report the H₂ mass density derived from the CO, using the CO(1−0) luminosity derived from Greve et al. (2014) and Sargent et al. (2014), and considering different α_CO values for star-forming galaxies, ULIRG-like galaxies, and dwarfs (solid blue line). We show the molecular gas mass density obtained by considering a single α_CO value for all the galaxies (dotted blue line), or two different α_CO values for star-forming and ULIRG-like galaxies (dashed blue line). We report observational results by Decarli et al. (2016, 2019, 2020), Saintonge et al. (2017), Riechers et al. (2019), and Magnelli et al. (2020, from the evolution of the dust mass density). The shaded areas include the 1σ uncertainties on the input LF, the high-z extrapolations, and the H₂ conversions. For the sake of clarity, we report uncertainties only for the results obtained from [C II] and from CO, assuming three different α_CO values for star-forming galaxies, ULIRG-like galaxies, and dwarfs. The uncertainties associated with other results derived from CO, under different α_CO assumptions, are comparable.