Evolution of T_d as a function of redshift. We include stacked MS galaxies from Schreiber et al. (2018) (the small red circles present the stacked galaxies in the largest mass bin with 11.0 <  log(M_⋆/M_⊙)< 11.5 whereas large red filled circles are the weighted mean of all galaxies), stacked MS and SB galaxies from Béthermin et al. (2015) (open red and blue symbols, respectively). For the latter, we convert ⟨U⟩ to T_d following Schreiber et al. (2018). We also include four dusty SFGs from Jin et al. (2019) (open blue triangles). Purple symbols depict the derived T_ex of GN20 from the [C I] luminosity ratio and from the MBB modeling assuming optically thin or thick FIR dust emission.