OCO-2 v10 MIP CO2 flux evaluation using an independent atmospheric chemistry-transport model

A recent model intercomparison project in carbon cycle community, OCO-2 v10 MIP, as detailed in Byrne et al., 2023, addresses the uncertainty in estimating CO₂ budgets at a hemispheric scale using the top-down approach. OCO-2 v10 MIP employs various atmospheric chemistry-transport models (ACTMs) and utilizes long-term background station CO₂ data as well as Orbiting Carbon Observatory 2 (OCO-2) total column CO₂ (XCO₂) observations to provide CO₂ fluxes during 2015-2020. This study aims to evaluate these fluxes using an independent ACTM, MIRO4-ACTM (Chandra et al., 2022), to analyse semi-hemispheric CO₂ sources and sinks for a known distribution of fossil fuel emissions. Our analysis shows that 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>30^o). Utilising these correlations, we define emergent constraints to calculate “best estimate” total CO₂ fluxes. After correcting for ocean and riverine carbon export, land carbon stockchange during 2015-2020 are -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 align closely with those calculated from national inventories and land surface observations but suggest that atmospheric CO₂ inversion models generally overestimate extratropical carbon uptake and tropical emissions. To further validate, we compared our findings with the results using another ACTM, GEOS-CHEM (participated in OCO-2 v10 MIP), demonstrating similar agreement.