Measured CO luminosities, corrected for amplification when known, but not for gas excitation, as a function of redshift. We compare our points (filled black circles, and arrows as upper limits) with a compilation of high-z molecular gas surveys, and local ones: green filled triangles are from Gao & Solomon (2004), filled blue squares from Solomon et al. (1997), open circles from Chung et al. (2009), open blue diamonds from Geach et al. (2009, 2011), black crosses from Iono et al. (2009), red stars from Greve et al. (2005), green filled circles from Daddi et al. (2010), blue asterisks from Genzel et al. (2010), and blue filled circles from Solomon & vanden Bout (2005). For illustration purposes only, the red curve is the power law in (1 + z)^1.6 for Ω_H2/Ω_HI proposed by Obreschkow & Rawlings (2009).