A new diagnostic reveals how fluxes constrained by two different CO₂ observing systems inform atmospheric CO₂ simulations. The potential for GOSAT data to better resolve zonally asymmetric structures in the tropics year-round and in the northern extratropics in most seasons is shown. Using in situ data yields a better match to independent observations on the global, annual scale. Such complementarity of the observing systems can be exploited in greenhouse gas data assimilation systems.