The use of global three-dimensional (3-D) models with satellite observations of CO<sub>2</sub> in inverse modeling studies is an area of growing importance for understanding Earth's carbon cycle. Here we use the GEOS-Chem model (version 8-02-01) CO<sub>2</sub> simulation with multiple modifications in order to assess their impact on CO<sub>2</sub> forward simulations. Modifications include CO</sub>2</sub> surface emissions from shipping (~0.19 Pg C/yr), 3-D spatially-distributed emissions from aviation (~0.16 Pg C/yr), and 3-D chemical production of CO<sub>2</sub> (~1.05 Pg C/yr). Although CO<sub>2</sub> chemical production from the oxidation of CO, CH<sub>4</sub> and other carbon gases is recognized as an important contribution to global CO<sub>2</sub>, it is typically accounted for by conversion from its precursors at the surface rather than in the free troposphere. We base our model 3-D spatial distribution of CO<sub>2</sub> chemical production on monthly-averaged loss rates of CO (a key precursor and intermediate in the oxidation of organic carbon) and apply an associated surface correction for inventories that have counted emissions of carbon precursor as CO<sub>2</sub>. We also explore the benefit of assimilating satellite observations of CO into GEOS-Chem to obtain an observation-based estimate of the CO<sub>2</sub> chemical source. The CO assimilation corrects for an underestimate of atmospheric CO abundances in the model, resulting in increases of as much as 24% in the chemical source during May–June 2006, and increasing the global annual estimate of CO<sub>2</sub> chemical production from 1.05 to 1.18 Pg C. Comparisons of model CO<sub>2</sub> with measurements are carried out in order to investigate the spatial and temporal distributions that result when these new sources are added. Inclusion of CO<sub>2</sub> emissions from shipping and aviation are shown to increase the global CO<sub>2</sub> latitudinal gradient by just over 0.10 ppm (~3%), while the inclusion of CO<sub>2</sub> chemical production (and the surface correction) is shown to decrease the latitudinal gradient by about 0.40 ppm (~10%) with a complex spatial structure generally resulting in decreased CO</sub>2</sub> over land and increased CO<sub>2</sub> over the oceans. Since these CO<sub>2</sub> emissions are omitted or misrepresented in most inverse modeling work to date, their implementation in forward simulations should lead to improved inverse modeling estimates of terrestrial biospheric fluxes.