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This work estimates global-mean K_zz using Sounding of the Atmosphere using Broadband Emission Radiometry/Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics monthly global-mean CO₂ profiles and a one-dimensional transport model. It is then specified as a lower boundary into the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM). Results first show that global-mean CO₂ in the mesosphere and lower thermosphere region has annual and semiannual oscillations (AO and SAO) with maxima during solstice seasons along with a primary maximum in boreal summer. Our calculated AO and SAO in global-mean CO₂ are then modeled by AO and SAO in global-mean K_zz. It is then shown that our estimated global-mean K_zz is lower in magnitude than the suggested global-mean K_zz from Qian et al. (2009) that can model the observed AO and SAO in the ionosphere/thermosphere (IT) region. However, our estimated global-mean K_zz is similar in magnitude with recent suggestions of global-mean K_zz in models with explicit gravity wave parameterization. Our work therefore concludes that global-mean K_zz from global-mean CO₂ profiles cannot model the observed AO and SAO in the IT region because our estimated global-mean K_zz may only be representing eddy diffusion due to gravity wave breaking. The difference between our estimated global-mean K_zz and the global-mean K_zz from Qian et al. (2009) thus represents diffusion and mixing from other nongravity wave sources not directly accounted for in the TIE-GCM lower boundary conditions. These other sources may well be the more dominant lower atmospheric forcing behind the AO and SAO in the IT region.