Improved Constraints on the Recent Terrestrial Carbon Sink Over China by Assimilating OCO-2 XCO2 Retrievals

The magnitude and distribution of China's terrestrial carbon sink remain uncertain due to insufficient observational constraints; satellite column-average dry-air mole fraction carbon dioxide (XCO2) retrievals may fill some of this gap. Here, we estimate China's carbon sink using atmospheric inversions of the Orbiting Carbon Observatory 2 (OCO-2) XCO2 retrievals within different platforms, including the Global Carbon Assimilation System (GCAS) v2, the Copernicus Atmosphere Monitoring Service, and the OCO-2 Model Inter-comparison Project (MIP). We find that they consistently place the largest net biome production (NBP) in the south on an annual basis compared to the northeast and other main agricultural areas during peak growing season, coinciding well with the distribution of forests and crops, respectively. Moreover, the mean seasonal cycle amplitude of NBP in OCO-2 inversions is obviously larger than that of biosphere model simulations and slightly greater than surface CO2 inversions. More importantly, the mean seasonal cycle of the OCO-2 inversions is well constrained in the temperate, tropical, and subtropical monsoon climate zones, with better inter-model consistency at a sub-regional scale compared to in situ inversions and biosphere model simulations. In addition, the OCO-2 inversions estimate the mean annual NBP in China for 2015-2019 to be between 0.34 (GCASv2) and 0.47 +/- 0.16 PgC/yr (median +/- std; OCO-2 v10 MIP), and indicate the impacts of climate extremes (e.g., the 2019 drought) on the interannual variations of NBP. Our results suggest that assimilating OCO-2 XCO2 retrievals is crucial for improving our understanding of China's terrestrial carbon sink regime. Plain Language Summary The magnitude and distribution of China's terrestrial carbon sink remain underconstrained; satellite column-average dry-air mole fraction carbon dioxide (XCO2) retrievals from NASA's Carbon Observatory 2 (OCO-2) could help reduce this uncertainty. This study revisited China's terrestrial carbon sink estimates based on state-of-the-art OCO-2 XCO2 inversions, including the Global Carbon Assimilation System OCO-2 inversion, the Copernicus Atmosphere Monitoring Service OCO-2 inversion, and those in the OCO-2 Model Inter-comparison Project. We found that the assimilation of OCO-2 XCO2 retrievals offers effective constraints on the spatiotemporal patterns of the terrestrial carbon sink of China. This result suggests that the OCO-2 XCO2 inversions allow an improved understanding of China's land carbon sink over in situ CO2 inversions and bottom-up biosphere model simulations, including better representations in spatial distributions and seasonal cycles and more plausible interannual variations. These improvements suggest that the assimilation of OCO-2 XCO2 retrievals offers effective constraints on the spatiotemporal patterns of the terrestrial carbon sink of China.