A ten-year (2010-2019) global terrestrial NEE inferred from the GOSAT v9 XCO2 retrievals (GCAS2021)

Top-down atmospheric inversion is one of the major methods to estimate the global NEE. Here, by assimilating the GOSAT ACOS v9 XCO₂ product, we generate a ten-year (2010–2019) global monthly terrestrial NEE dataset using the Global Carbon Assimilation System, version 2 (GCASv2), which is named as GCAS2021. It includes(1) monthly and annual prior and posterior NEE and OCN fluxes, and prescribed FIRE and FFC emissions in a global spatial resolution of 1°×1°; (2) globally, latitudinally, and regionally aggregated monthly and annual posterior NEE and NBE, and their uncertainties; and (3) weekly gridded ensemble members of posterior NEE and OCN fluxes. The regional fluxes are aggregated both in the TRANSCOM and RECCAP2 regions. The latitudinal fluxes are aggregated in northern mid-high latitudes (> 30° N, NL), low latitudes (30° S ~ 30° N, TL), and southern middle latitudes (<30° N, SL). The weekly grided ensemble members could be used for calculating the posterior uncertainties of the user's area and time scale. 

Combining the OCN flux, and FIRE and FFC emissions, the net biosphere flux (NBE) and atmospheric growth rate (AGR) as well as their inter-annual variabilities (IAVs) agree well with the estimates of Global Carbon Budget 2020. Regionally, GCAS2021 shows that eastern North America, Amazon, Congo Basin, Europe, boreal forests, southern China and Southeast Asia are carbon sinks, while western US, African grasslands, Brazilian plateaus and parts of South Asia are carbon sources. In the TRANSCOM land regions, the NBEs of temperate N. America, northern Africa and boreal Asia are between the estimates of CMS-Flux NBE 2020 and CT2019B, and those in temperate Asia, Europe, and Southeast Asia are consistent with CMS-Flux NBE 2020 but significantly different from CT2019B. In the RECCAP2 regions, except for Africa and South Asia, the NBEs are comparable with the latest bottom-up estimate of Ciais et al. (2021). Compared with previous studies, the IAVs and seasonal cycles of NEE of this dateset could clearly reflect the impacts of extreme climates and large-scale climate anomalies on the carbon flux. The evaluations also show that the posterior CO₂ concentrations at remote sites and in regional scale, as well as on vertical CO₂ profiles in the Asia-Pacific region and the Amazon basin, are all consistent with independent CO₂ measurements from surface flask and aircraft CO₂ observations, indicating that this dataset captures surface carbon fluxes well. We believe that this data set will contribute to regional or national-scale carbon cycle and carbon neutrality assessment, and carbon dynamics research.