Constraint of satellite CO2 retrieval on the global carbon cycle from a Chinese atmospheric inversion system

Satellite carbon dioxide (CO2) retrievals provide important constraints on surface carbon fluxes in regions that are undersampled by global in situ networks. In this study, we developed an atmospheric inversion system to infer CO2 sources and sinks from Orbiting Carbon Observatory-2 (OCO-2) column CO2 retrievals during 2015-2019, and compared our estimates to five other state-of-the-art inversions. By assimilating satellite CO2 retrievals in the inversion, the global net terrestrial carbon sink (net biome productivity, NBP) was found to be 1.03 +/- 0.39 petagrams of carbon per year (PgC yr(-1)); this estimate is lower than the sink estimate of 1.46-2.52 PgC yr(-1), obtained using surface-based inversions. We estimated a weak northern uptake of 1.30 PgC yr(-1) and weak tropical release of -0.26 PgC yr(-1), consistent with previous reports. By contrast, the other inversions showed a strong northern uptake (1.44-2.78 PgC yr(-1)), but diverging tropical carbon fluxes, from a sink of 0.77 PgC yr(-1) to a source of -1.26 PgC yr(-1). During the 2015-2016 El Nino event, the tropical land biosphere was mainly responsible for a higher global CO2 growth rate. Anomalously high carbon uptake in the northern extratropics, consistent with concurrent extreme Northern Hemisphere greening, partially offset the tropical carbon losses. This anomalously high carbon uptake was not always found in surface-based inversions, resulting in a larger global carbon release in the other inversions. Thus, our satellite constraint refines the current understanding of flux partitioning between northern and tropical terrestrial regions, and suggests that the northern extratropics acted as anomalous high CO2 sinks in response to the 2015-2016 El Nino event.

Automated summary

Satellite carbon dioxide retrievals have provided important constraints on surface carbon fluxes in undersampled regions. This study developed an atmospheric inversion system to estimate CO2 sources and sinks using Orbiting Carbon Observatory-2 (OCO-2) column CO2 retrievals from 2015 to 2019, and compared the estimates with other state-of-the-art inversions. Assimilating satellite CO2 retrievals, the global net terrestrial carbon sink (net biome productivity, NBP) was estimated to be 1.03 +/- 0.39 petagrams of carbon per year (PgC yr(-1)), lower than estimates from surface-based inversions. The results showed a weak northern uptake and weak tropical release of carbon, consistent with previous reports. In contrast, the other inversions showed a strong northern uptake but varied tropical carbon fluxes. During the 2015-2016 El Nino event, the tropical land biosphere played a significant role in global CO2 growth rate, while the anomalously high carbon uptake in the northern extratropics partially offset the tropical carbon losses.