The spectral absorption coefficient, α(λ) [m⁻¹], is an optical property that quantifies how strongly a material absorbs radiation at a given wavelength (or wavenumber). Knowledge of the CO₂ absorption coefficient is essential for modeling thermal radiative heat transfer in the atmosphere. In this work, we report α(λ) and its spectrally averaged value a for: (i) pure CO₂, (ii) CO₂-air mixtures, and (iii) CO₂-argon mixtures. In the mixtures, the CO₂ concentration was doubled, quadrupled, and increased up to fifteen times relative to the presentday concentration in the atmosphere The spectral absorptivity A(λ) was measured in a 1.8 m long test tube at normal temperature and pressure using a Nicolet FTIR spectrometer, with 0.5 cm-1 resolution. We focused on the 15 μm band (wavenumbers 800 cm-1 to 600 cm-1) because is the only CO₂ band that can trap the heat emitted by Earth. The measured spectra show that doubling the CO₂ concentration from 420 ppm to 840 ppm increases the averaged absorption coefficient a by approximately 20 %. A drastic increase in α(λ) was observed when air molecules were added to pure CO₂, due to Vibrational-Translational (V-T) de-excitation by collisions. A similar increase was demonstrated when Argon atoms were added to pure CO₂.