Measurements of the CMB temperature as a function of redshift. Data points in green correspond to S-Z measurements toward galaxy clusters, in black to C i and C ii absorption studies, in blue to CO absorption (see Noterdaeme et al. 2011 and references therein), with the value derived toward the PKS 1830−211 SW absorption marked in red (this paper). The dotted line corresponds to the law T_CMB = T₀ × (1 + z).