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.

自上而下大气反演（Top-down atmospheric inversion）是估算全球净生态系统交换（Net Ecosystem Exchange, NEE）的主要方法之一。本研究通过同化GOSAT ACOS v9 XCO2产品，利用全球碳同化系统第2版（Global Carbon Assimilation System, version 2, GCASv2）构建了2010–2019年十年期全球陆地月尺度净生态系统交换数据集，命名为GCAS2021。该数据集包含以下内容：(1) 全球1°×1°空间分辨率下的月、年尺度先验与后验净生态系统交换通量、有机碳循环（Organic Carbon Cycle, OCN）通量，以及预设的野火（FIRE）与化石燃料燃烧（FFC）排放数据；(2) 全球、纬向及区域汇总的月、年尺度后验净生态系统交换与净生物圈通量（Net Biosphere Exchange, NBE）及其不确定性；(3) 后验净生态系统交换与有机碳循环通量的周尺度格点集合成员。区域通量按TRANSCOM区域与RECCAP2区域进行汇总。纬向通量则按北中高纬（>30°N，NL）、低纬（30°S ~ 30°N，TL）及南中纬（<30°N，SL）进行汇总。周尺度格点集合成员可用于计算用户指定区域与时间尺度的后验不确定性。

结合有机碳循环通量、野火与化石燃料燃烧排放数据，本数据集得到的净生物圈通量（NBE）、大气CO₂增长率（AGR）及其年际变率（Inter-Annual Variabilities, IAVs）与《2020年全球碳预算》（Global Carbon Budget 2020）的估算结果吻合良好。区域尺度上，GCAS2021数据集显示北美东部、亚马逊平原、刚果盆地、欧洲、北方森林、中国南部与东南亚为碳汇，而美国西部、非洲草原、巴西高原及南亚部分区域为碳源。在TRANSCOM陆地区域中，温带北美、北非与寒带亚洲的净生物圈通量介于CMS-Flux 2020与CT2019B的估算结果之间；而温带亚洲、欧洲与东南亚的净生物圈通量与CMS-Flux 2020的结果一致，但与CT2019B的结果存在显著差异。在RECCAP2区域中，除非洲与南亚外，其余区域的净生物圈通量与Ciais等（2021）最新的自下而上估算结果相当。与既往研究相比，本数据集的净生态系统交换年际变率与季节循环可清晰反映极端气候与大尺度气候异常对碳通量的影响。验证结果显示，偏远站点与区域尺度的后验CO₂浓度，以及亚太区域与亚马逊盆地的垂直CO₂廓线，均与基于地面钢瓶采样与航空器观测得到的独立CO₂测量数据吻合良好，表明本数据集可较好地捕捉地表碳通量特征。本数据集有望为区域或国家级碳循环与碳中和评估以及碳动力学研究提供有力支撑。