Atmospheric CO2 inversion models overestimate northern extratropical land and ocean carbon uptake as assessed at background in-situ sites

Abstract Several aspects of the global biospheric carbon cycle are highly uncertain, including estimation of CO2 budgets at hemispheric scale by top-down inverse modelling approach. We use CO2 fluxes from a model intercomparison project (OCO-2_v10_MIP) that was conducted using long-term background station CO2 and total-column CO2 (XCO2) observations for the period 2015–2020. Total (ocean + land) CO2 flux and mean model-observed CO2 differences against 50 background sites exhibit statistically significant correlations for the northern and southern extra-tropics (latitude > 30o). Using these correlations, we define emergent constraints to calculate “best estimate” total CO2 fluxes. After correcting ocean fluxes and riverine carbon export, we estimate terrestrial CO2 fluxes or land carbon stockchange of -1.54 ± 1.18, -0.04 ± 1.11 and − 0.13 ± 0.27 PgC yr− 1 in the northern extra-tropics, tropics, and southern extra-tropics, respectively. These fluxes agree well with those based on national inventories and land surface observations, but suggest that CO2 inversion models generally overestimate northern extratropical uptake and tropical emissions.