To date, the measurements from AIRS, GOSAT, SCIAMACHY and IASI are widely used to derive atmospheric CO<sub>2</sub> concentration globally. Quantification of the spatio-temporal differences between these CO<sub>2</sub> products is crucial for deeply understanding each product and for exploring the potential for their joint use in the future. A series of processing has been proposed in this paper to unify these existing CO<sub>2</sub> products, so that they can be theoretically comparable. Based on this, the spatio-temporal variations of these CO<sub>2</sub> products have been conducted. The results reveal that AIRS show the largest spatial coverage of CO<sub>2</sub> compared to other CO<sub>2</sub> products. The valid X<sub>CO2</sub> data from SCIAMACHY are mainly restricted to land regions. Even if over the land, the X<sub>CO2 </sub>distribution of ACOS is slightly wider than that of SCIAMACHY, especially over the region of Eurasia and North America. An obvious arch-shaped pattern along latitude for ACOS, Japan-GOSAT and SCIAMACHY is detected, while no distinct latitudinal variation can be observed for AIRS. For the seasonal variation, these datasets show a similar trend with the maximum CO<sub>2</sub> loading occurring in spring. The discrepancy between these products implies that it is greatly necessary to better constrain the uncertainties in CO<sub>2</sub> retrieval from space in the future.