A Top-Down View of Global and Regional Carbon Budgets From an Ensemble of Atmospheric Inversions

Abstract

Atmospheric inversions provide surface CO₂ flux estimates based on in situ observed atmospheric CO₂ mole fractions or satellite-based column average CO₂ (XCO₂). Here, we provide a detailed assessment of 14 atmospheric CO₂ inversions included in the Global Carbon Budget (GCB2024). We develop tools to further assess and use these inversions in global and regional carbon cycle studies including the GCB and the REgional Carbon Cycle Assessment and Processes (RECCAP2) initiative. We show that the global atmospheric CO₂ growth rate and its interannual variability are reproduced well by all inversions. In contrast to bottom-up models, inversions provide carbon flux estimates directly constrained by observations. Our ensemble mean estimates of the global sinks for the period 2015–2023 are −1.41$\pm$0.55 PgC yr⁻¹ for the net land sink (including land-use change emissions) and −2.97$\pm$0.55 PgC yr⁻¹ for the global net ocean sink (uncertainties reported as $\pm 1\sigma$ across inversions; estimates include fossil fuel and river flux adjustments). On regional scales, we find significant spread in flux estimates between inversions across regions, and we present criteria and metrics to derive flux-observation relation constraints or subselect ensembles of inversions based on independent observations. Furthermore, we use the atmospheric inversions to assess the atmospheric growth rate of CO₂. We show that the factor used to convert annual observation-based growth rates to net fluxes is variable over time as a result of atmospheric mixing. Finally, we propose guidelines on how to use the inverse results in global and regional carbon budget studies by the wider carbon cycle community.