Top: effect of cosmic variance based purely on the survey size. We show the results of the molecular gas mass density variation derived from the SIDES simulation (Béthermin et al. 2022; Gkogkou et al. 2022), publicly available for J = 2 at redshifts z = 0.5–6. The solid blue line indicates the median value of ρ_H2, and the dark and light areas represent the 1σ and 2σ variation due to the cosmic variance. We plot the area covered by ALMACAL-22 along with other CO surveys that have measured the molecular gas mass density. Overall, ALMACAL-22 is the survey less affected by cosmic variance to date. Bottom: cosmic variance estimation of ALMACAL-22 survey in purple, based on Driver & Robotham 2010 (D&R+10), for different CO transitions (Table 1). We also plot the variance derived from the formula presented by Keenan et al. (2020) (K+20), which agrees with the previous prescription. The figure shows estimates from ASPECS (Decarli et al. 2019, 2020), PHIBSS (Lenkić et al. 2020), COLDz GOODS and COSMOS (Pavesi et al. 2018; Riechers et al. 2019), previously reported in Lenkić et al. (2020). For every CO transition, ALMACAL-22 has the lowest effect of field-to-field variance (≲5%).