Toward a long-term atmospheric CO2 inversion for elucidating natural carbon fluxes: technical notes of NISMON-CO2 v2021.1

Abstract Global estimates of carbon dioxide (CO 2 ) fluxes at the earth’s surface are imperative to understanding carbon cycle mechanisms and making reliable predictions about the global warming. Furthermore, they can provide valuable science-based information for use in reducing human-induced CO 2 emissions. Inverse analysis is a prominent method for quantitatively estimating spatiotemporal variations in CO 2 fluxes; however, this technique involves a certain degree of uncertainty and requires technical refinement, especially to improve the horizontal resolution so that local fluxes can be compared with other estimates made at the regional or national level. In this study, a newly developed set of inversion schemes was incorporated into a state-of-the-art inverse analysis system named NISMON-CO 2 . The introduced schemes involve grid conversion, observational weighting, and anisotropic prior error covariance, the details of which are described in this paper. In addition, pseudo-observation experiments were performed to investigate the effect of the new schemes, as well as to assess the reliability of NISMON-CO 2 for long-term analysis with practical inhomogeneous observations. The experiment results demonstrate the utility of the grid conversion scheme for high-resolution flux estimates (1° × 1°), and significant improvements were achieved through the observational weighting and anisotropic prior error covariance. Furthermore, the estimated seasonal and interannual variations in regional CO 2 fluxes were confirmed to be reliable, although there is some possible bias in terms of global land–ocean partitioning. These results are useful for interpreting flux variations that result from inverse analysis by NISMON-CO 2 ver. 2021.1 using real observations.