Comment on essd-2022-412

Abstract. Quantification of land surface-atmosphere fluxes of carbon dioxide (CO₂) fluxes and their trends and uncertainties is essential for monitoring progress of the EU27+UK bloc as it strives to meet ambitious targets determined by both international agreements and internal regulation. This study provides a consolidated synthesis of fossil sources (CO₂ fossil) and natural sources and sinks over land (CO₂ land) using bottom-up (BU) and top-down (TD) approaches for the European Union and United Kingdom (EU27+UK), updating earlier syntheses (Petrescu et al., 2020, 2021b). Given the wide scope of the work and the variety of approaches involved, this study aims to answer essential questions identified in the previous syntheses and understand the differences between datasets, particularly for poorly characterized fluxes from managed ecosystems. The work integrates updated emission inventory data, process-based model results, data-driven sectoral model results, and inverse modeling estimates, extending the previous period 1990–2018 to the year 2020 to the extent possible. BU and TD products are compared with European National Greenhouse Gas Inventories (NGHGIs) reported by Parties including the year 2019 under the United Nations Framework Convention on Climate Change (UNFCCC). The uncertainties of the EU27+UK NGHGI were evaluated using the standard deviation reported by the EU Member States following the guidelines of the Intergovernmental Panel on Climate Change (IPCC) and harmonized by gap-filling procedures. Variation in estimates produced with other methods, such as atmospheric inversion models (TD) or spatially disaggregated inventory datasets (BU), originate from within-model uncertainty related to parameterization as well as structural differences between models. By comparing NGHGIs with other approaches, key sources of differences between estimates arise primarily in activities. System boundaries and emission categories create differences in CO₂ fossil datasets, while different land use definitions for reporting emissions from Land Use, Land Use Change and Forestry (LULUCF) activities result in differences for CO₂ land. The latter has important consequences for atmospheric inversions, leading to inversions reporting stronger sinks in vegetation and soils than are reported by the NGHGI. For CO₂ fossil emissions, after harmonizing estimates based on common activities and selecting the most recent year available for all datasets, the UNFCCC NGHGI for the EU27+UK accounts for 3392 ± 49 Tg CO₂ yr^-1 (926 ± 13 Tg C yr^-1), while eight other BU sources report a mean value of 3340 [3238,3401] [25th,75th percentile] Tg CO₂ yr^-1 (948 [937,961] Tg C yr^-1). The sole top-down inversion of fossil emissions currently available accounts for 3800 Tg CO₂ yr^-1 (1038 Tg C yr^-1), a value close to that of the NGHGI, but for which uncertainty estimates are not yet available. For the net CO₂ land fluxes, during the most recent five-year period including the NGHGI estimates, the NGHGI accounted for -91 ± 32 Tg C yr^-1 while six other BU approaches reported a mean sink of -62 [-117,-49] Tg C yr^-1 and a 15-member ensemble of dynamic global vegetation models (DGVMs) reported -69 [-152,-5] Tg C yr^-1. The five-year mean of three TD regional ensembles combined with one non-ensemble inversion of -73 Tg C yr^-1 has a slightly smaller spread (0th–100th percentile of [-135,45] Tg C yr^-1), and was calculated after removing land-atmosphere CO₂ fluxes caused by lateral transport of carbon (crops, wood trade and inland waters) resulting in increased agreement with the the NGHGI and bottom-up approaches. Results at the sub-sector level (Forestland, Cropland, Grassland) show generally good agreement between the NGHGI and sub-sector-specific models, but results for a DGVM are mixed. Overall, for both CO₂ fossil and net CO₂ land fluxes, we find current independent approaches are consistent with the NGHGI at the scale of the EU27+UK. We conclude that CO₂ emissions from fossil sources have decreased over the past 30 years in the EU27+UK, while large uncertainties on net uptake of CO₂ by the land surface prevent trend identification. In addition, a gap on the order of 1000 Tg C yr^-1 between CO₂ fossil emissions and net CO₂ uptake by the land exists regardless of the type of approach (NGHGI, TD, BU), falling well outside all available estimates of uncertainties. However, uncertainties in top-down approaches to estimate CO₂ fossil emissions remain uncharacterized and are likely substantial. The data used to plot the figures are available at https://doi.org/10.5281/zenodo.7365863.