Molecular-hydrogen fraction as a function of metallicity for the H₂-bearing GRB absorbers (red). Filled squares denote GRBs where C I is also detected, whereas empty squares represent GRBs with H₂ but non-detections of C I. The statistical sample of quasar absorbers from Ledoux et al. (2003) are shown as the small blue symbols where circles denote absorbers with H₂ detections and triangles show the upper limits on the molecular-hydrogen fractions of the absorbers with non-detections of H₂. Filled blue symbols denote quasar absorbers where Srianand et al. (2005) detected C I in absorption, whereas empty blue symbols represent quasar absorbers with non-detections of C I. For comparison, the C I-selected quasar absorbers with measurements of C I, H₂ (Noterdaeme et al. 2018) and metallicities from the literature, are shown as the filled blue diamond symbols. The large blue dots represent the same ES-DLAs as shown in Fig. 10.