Estimates of Polar Ocean CO2 Uptake from Atmospheric Inverse Analyses

Estimates of global scale air-sea CO₂ fluxes have traditionally been derived from ocean biogeochemistry models and ocean surface pCO₂ data products (Friedlingstein et al. 2022). An alternative means of estimating ocean carbon uptake is provided by atmospheric inversions; these use optimization procedures and data assimilation methods to combine atmospheric CO₂ measurements with numerical transport model simulations and prior knowledge of air-sea fluxes. 

Here we use the GEOSChem-LETKF (GCLETKF) inverse system (Chen et al. 2021) in conjunction with atmospheric observations from the NOAA-GML surface CO₂ measurement network to derive grid-scale air-sea CO₂ flux estimates for the period 2000-2017. We focus, in particular, on estimates of CO₂ uptake by the polar oceans (Southern and Arctic oceans). These  regions have accounted for a significant component of global oceanic carbon uptake  in recent decades (e.g., more than 20% of global ocean uptake, in comparison to their ocean areal  extent of < 10%).

We present GCLETKF estimates of ocean CO₂ uptake at global and regional scales, and assess the robustness of our results with a suite of metrics that include model concentration bias, CO₂ flux error reduction, and comparison to independent atmospheric measurements. GCLETKF flux estimates for the 2000-2017 period indicate regional CO₂  uptake of 0.1-0.2 PgC/year for the Arctic,  and  0.45-0.55 PgC/yr for the Southern Ocean. We also provide summary estimates of the  interannual variations and  decadal-scale trends of the polar ocean carbon fluxes, and compare the GCLETKF results  to estimates derived from global ocean biogeochemistry models and surface ocean pCO₂ data products.