Top: cosmic variance in the ρ_H2 values as a function of the survey size for different redshift slices. The solid and dashed lines are the mean and median values, respectively. We have also included the corresponding results from Keenan et al. (2020) (the shaded and hatched areas). Bottom: evolution with redshift of the variance in the molecular gas mass density as a function of survey sizes, where it is easier to spot what is the proper survey size that will allow to probe the evolution of the molecular gas density.