Combined CO₂ measurement record indicates decreased Amazon forest carbon uptake, offset by Savannah carbon release

Abstract. In tropical South America there has been substantial progress on atmospheric monitoring capacity, but the region still has a limited number of continental atmospheric stations relative to its large area, hindering net carbon flux estimates using atmospheric inversions. In this study, we use dry air CO₂ mole fractions measured at the Amazon Tall Tower Observatory (ATTO) and airborne vertical CO₂ profiles in an atmospheric inversion system to estimate the net carbon exchange in tropical South America. We focus on the biogeographic Amazon, and its neighboring Cerrado and Caatinga biomes. Considering all prior ensemble members, we estimate that the biogeographic Amazon was a net carbon sink with the sum of vegetation uptake, river outgassing and carbon release from fires at a median of -0.33 ± 0.33 PgC year^-1. Using only process-based models as input in the inversion system the uptake is reduced to -0.24 ± 0.33 PgC year^-1. The Cerrado and Caatinga biomes together represent a median carbon source of 0.31 ± 0.24 PgC year^-1, with contributions from both vegetation carbon release and fires. Therefore, we estimate that the net carbon balance for tropical South America is close to neutral, but we note that the uncertainties straddle zero net exchange. In addition, we calculate the effect of systematic uncertainties in the inverse estimates by proposing a water-vapor correction to measured airborne CO₂ profiles. Finally, to further reduce the uncertainty in regional carbon balance estimates in tropical South America, we call for an expansion of the atmospheric monitoring network on the continent, mainly in the Amazon-Andes foothills.