Predicted average luminosity ratios for MS galaxies of various CO transitions normalized to L′CO[1–0], exploiting the correlation between CO SLED excitation and ⟨ U ⟩ and between ⟨ U ⟩ and redshift (Magdis et al. 2012; Béthermin et al. 2015). This is computed for the J = 2,3,4,5 transitions, curves from top to bottom. We considered two-component cold and warm CO SLEDs, corresponding to the different solutions shown in Fig. 10, with the warmer component peaking at J = 5 (solid) or 7 (dashed).