Field-to-field variance of the molecular gas mass density, ρ_H2, at several redshift slices where observational data are available. Left: evolution of ρ_H2 with redshift. The empty boxes are the observational data from ASPECS (Decarli et al. 2020) and COLDz (Riechers et al. 2019). We also include the upper limit offered by COLDz assuming that all the galaxies could be CO(2–1) emitters at high z. The black dashed line is the SIDES model without a luminosity cut and the black dotted line is the SIDES model with a luminosity cut at ∼5 × 10⁹ K km s⁻¹ pc². The steps in both lines are caused by the selected redshift grid that the cosmological simulation snapshots were taken. The shaded gray areas correspond to the ρ_H2 field-to-field variance which has been computed for the same redshift ranges and sizes as the corresponding surveys. Right: field-to-field variance in the molecular gas density in different redshift bins. The light-colored areas show the total field-to-field variance introduced in the molecular gas density estimates, while the dark-colored areas show the Poisson-only variance. The contribution of the latter to the total variance is given as a percentage on top of each rectangle. The different colors indicate the different surveys, hence different size and redshift slice.