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Atmospheric inversion estimates of net ecosystem exchange (NEE) of CO2 are increasingly relevant to climate policy. We evaluated sub‐continental, seasonal estimates of CO2 NEE from nine global inversion systems that participated in the Orbiting Carbon Observatory‐2 model intercomparison project (OCO‐2 v9 MIP), using 98 research flights conducted over the central and eastern United States from 2016 to 2018 as part of the Atmospheric Carbon and Transport ‐ America mission. We found that the seasonal amplitude of NEE in the central and eastern United States is underestimated in these models and model‐data biases are largest for those inversions with the smallest seasonal flux amplitudes. These results were independent of whether the inversions used satellite or in situ data. The largest NEE biases were observed in the Midwest croplands and eastern forests. Future experiments are needed to determine the causes of the persistent biases and if they are associated with biases in annual flux estimates. Plain Language Summary: The exchange of CO2 between terrestrial ecosystems and the atmosphere is an important component of the Earth's climate system. Atmospheric budgets are used to quantify this exchange globally, but these estimates are difficult to evaluate on a regional basis. We used a unique set of aircraft data to evaluate a set of state‐of‐the‐science estimates of ecosystem‐atmosphere CO2 exchange in temperate North America. Nearly every estimate underestimated the seasonal amplitude of ecosystem‐atmosphere CO2 exchange (net photosynthesis too weak in the summer; respiration too weak in the winter) in this region. The source of atmospheric CO2 data did not influence this finding. More study is needed to determine both the cause of these seasonal biases and the impact of this bias on annual net CO2 flux estimates. Key Points: The seasonal amplitude of net ecosystem exchange (NEE) of CO2 in the central and eastern temperate North America is underestimated in global atmospheric inversionsThe seasonal bias is not significantly different between inversions using OCO‐2 v9 land nadir/glint observations and in situ observationsThe largest NEE biases are observed in U.S. croplands and eastern forests [ABSTRACT FROM AUTHOR]

The Atmospheric Carbon and Transport (ACT)-America NASA Earth Venture Suborbital Mission set out to improve regional atmospheric greenhouse gas (GHG) inversions by exploring the intersection of the strong GHG fluxes and vigorous atmospheric transport that occurs within the midlatitudes. Two research aircraft instrumented with remote and in situ sensors to measure GHG mole fractions, associated trace gases, and atmospheric state variables collected 1,140.7 flight hours of research data, distributed across 305 individual aircraft sorties, coordinated within 121 research flight days, and spanning five 6-week seasonal flight campaigns in the central and eastern United States. Flights sampled 31 synoptic sequences, including fair-weather and frontal conditions, at altitudes ranging from the atmospheric boundary layer to the upper free troposphere. The observations were complemented with global and regional GHG flux and transport model ensembles. We found that midlatitude weather systems contain large spatial gradients in GHG mole fractions, in patterns that were consistent as a function of season and altitude. We attribute these patterns to a combination of regional terrestrial fluxes and inflow from the continental boundaries. These observations, when segregated according to altitude and air mass, provide a variety of quantitative insights into the realism of regional CO2 and CH4 fluxes and atmospheric GHG transport realizations. The ACT-America dataset and ensemble modeling methods provide benchmarks for the development of atmospheric inversion systems. As global and regional atmospheric inversions incorporate ACT-America's findings and methods, we anticipate these systems will produce increasingly accurate and precise subcontinental GHG flux estimates. [ABSTRACT FROM AUTHOR]

Recent studies have raised a concern that the adverse impact of global virtual water trade on water depletion might pose potential risks to future food trade and water security. In this study, we investigate the patterns and drivers of agricultural virtual water imports in China to explore efficient measures to promote the sustainability of agricultural virtual water trade. The panel gravity model and the quantile regression are constructed in this research based on virtual water content and agricultural trade data of 245 categories of HS 6-bit encoded crop and animal products from 49 major exporters during 1998–2018. The main results are as follows: (1) Agricultural virtual water imports in China show a strong upward trend with an average annual growth rate of 13.09%, and oil products occupy the largest share. Both the number of import sources (extended margins) and the import volume from each county (intensive margins) show an increasing trend, but America makes the greatest contribution to virtual water imports of China. (2) Agricultural virtual water imports of China could well match the water resource endowment of exporters, and could also achieve net global water savings, which is beneficial to the sustainability of global water utilization. (3) Water and land resource endowments and economic scale of exporter as three essential supply factors have a positive impact on agricultural virtual water imports of China. Among demand factors, water demand of product and economic scale of China has a positive effect, while import price has a negative effect. Moreover, supply and demand factors have a stronger impact at higher quantiles. In addition, trade cost factors, including average tariff, bilateral exchange rate, and Free Trade Assignment (FTA) could also significantly affect agricultural virtual water imports. Data will be made available on request. • Agricultural virtual water imports in China show an upward trend, and oil and grain products occupy the largest share. • Virtual water imports could well match the water resource endowment of exporters and achieve net global water savings. • Water and land endowment, bilateral economic and trade costs are main drivers affecting virtual water imports. • The heterogenous impacts of drivers on virtual water imports are also investigated. [ABSTRACT FROM AUTHOR]