Carbon dioxide (${\mathrm{CO}}_{2}$) has been recently reported to possess an amorphous form, named ``carbonia,'' structurally similar to other group-IV oxide glasses. By combining ab initio constant pressure molecular dynamics, density-functional perturbation theory, and experimental IR spectra, we show that carbonia, and possibly also phase VI, is not ${\mathrm{SiO}}_{2}$-like, and that instead it is partially tetrahedral containing also a sizable amount of carbon in threefold coordination, but no sixfold octahedral coordination. Enthalpic considerations suggest that carbonia is a metastable intermediate state of the transformation of molecular ${\mathrm{CO}}_{2}$ into fully tetrahedral phases.