The magnitude and distribution of China's terrestrial carbon sink remain uncertain due to insufficient constraints at large scales, whereby satellite data offer great potential for reducing the uncertainty. Here, we present two carbon sink estimates for China constrained either by satellite CO 2 column concentrations (XCO 2 ) within the Global Carbon Assimilation System or by remotely sensed soil moisture and Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) in addition to in situ CO 2 observations within the Carbon Cycle Data Assimilation System. They point to a moderate size of carbon sinks of 0.34 ± 0.14 (mean ± unc.) and 0.43 ± 0.09 PgC/yr during 2010-2015, which are supported by an inventory-based estimate for forest and soil carbon sink (0.26 PgC/yr) and fall in the range of contemporary ensemble atmospheric inversions (0.25-0.48 PgC/yr). They also agree reasonably well on interannual variations, which reflect the carbon sink anomalies induced by regional droughts in southwest China. Furthermore, their spatial distributions are broadly consistent that of the forest inventory-based estimate, indicating that the largest carbon sinks locate in central and eastern China. Their estimates for forest carbon sink coincide fairly well with the inventory-based estimate across different regions, especially when aggregated to the north and south of China. Although enhanced recently by afforestation, China's carbon sink was also significantly weakened by regional droughts, which were often not fully represented in previous in situ CO 2 -based inversions due to insufficient observations. Our results suggest that satellite-based atmospheric CO 2 and land surface observations are vital in characterizing terrestrial net carbon fluxes at regional scales. Plain Language SummaryLimited by available in situ CO 2 observations, the size and geographical distribution of China's terrestrial carbon sink remain not well known, whereby satellite observations offer great potential for improving the situation. In this study, we estimate the carbon sink of China from either satellite CO 2 column concentrations (XCO 2 ) or remotely sensed land surface variables in addition to in situ CO 2 observations. They point to a moderate size of carbon sinks of 0.34 ± 0.14 (mean ± unc.) and 0.43 ± 0.09 PgC/ yr during 2010-2015. The size of China's carbon sink estimate is supported by an inventory-based estimate for forest and soil carbon sink (0.26 PgC/yr) and in line with contemporary atmospheric inversions (0.25-0.48 PgC/yr). They can detect the carbon sink anomalies induced by regional droughts in southwest China. Their estimates for forest carbon sink coincide fairly well with the inventory-based estimate across different regions, especially when aggregated to the north and south of China. China's carbon sink was significantly weakened by regional droughts, which were often not fully represented in previous in situ CO 2 -based inversions HE ET AL.
The European land carbon uptake has been heavily impacted by several recent severe droughts, yet quantitative estimates of carbon uptake anomalies are uncertain. Atmospheric CO 2 inverse models (AIMs) provide observation-based estimates of the large-scale carbon flux dynamics, but how well they capture drought impacts on the terrestrial carbon uptake is poorly known. Here we assessed the capacity of state-of-the-art AIMs in monitoring drought impacts on the European carbon uptake over 2001-2015 using observations of environmental variability and vegetation function and made comparisons with bottom-up estimates of carbon uptake anomalies. We found that global inversions with only limited surface CO 2 observations give divergent estimates of drought impacts. Regional inversions assimilating denser CO 2 observations over Europe demonstrated some improved consistency, with all inversions capturing a reduction in carbon uptake during the 2012 drought. However, they failed to capture the reduction caused by the 2015 drought. Finally, we found that a set of inversions that assimilated satellite XCO 2 or assimilated environmental variables plus surface CO 2 observations better captured carbon uptake anomalies induced by both the 2012 and 2015 droughts. In addition, the recent Orbiting Carbon Observatory-2 XCO 2 inversions showed good potential in capturing drought impacts, with better performances for larger-scale droughts like the 2018 drought. These results suggest that surface CO 2 observations may still be too sparse to fully capture the impact of drought on the carbon cycle at subcontinental scales over Europe, and satellite XCO 2 and ancillary environmental data can be used to improve observational constraints in atmospheric inversion systems.Plain Language Summary Atmospheric CO 2 inverse models (AIMs) are useful tools for quantifying the response of large-scale carbon uptake to climate extremes, but their capacity for monitoring drought impacts, particularly at regional scales, is not fully explored. In this study, we assessed the capacity of state-of-the-art AIMs for monitoring drought impacts on the European land carbon uptake over 2001-2015 using a large array of observational and model data sets. We found: (a) global inversions with only limited surface CO 2 observations face a great challenge in monitoring drought impacts on the European carbon uptake; (b) Regional inversions assimilated denser CO 2 observations over Europe, for the EUROCOM project, demonstrated some improved consistency but are still deficient, showing divergent estimates in interannual variability of carbon uptake for most years; and (c) A set of inversion systems that assimilated satellite XCO 2 or assimilated environmental variables plus surface CO 2 observations better captured annual and seasonal HE ET AL.
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