Supplementary material to "The consolidated European synthesis of CO₂ emissions and removals for EU27 and UK: 1990–2020"

Quantification of land surface-atmosphere fluxes of carbon dioxide (CO2) fluxes and their trends and 63 uncertainties is essential for monitoring progress of the EU27+UK bloc as it strives to meet ambitious targets 64 determined by both international agreements and internal regulation.This study provides a consolidated synthesis of 65 fossil sources (CO2 fossil) and natural sources and sinks over land (CO2 land) using bottom-up (BU) and top-down 66 (TD) approaches for the European Union and United Kingdom (EU27+UK), updating earlier syntheses (Petrescu et 67 al., 2020(Petrescu et 67 al., , 2021b)).Given the wide scope of the work and the variety of approaches involved, this study aims to answer 68 essential questions identified in the previous syntheses and understand the differences between datasets, particularly 69 for poorly characterized fluxes from managed ecosystems.The work integrates updated emission inventory data, 70 process-based model results, data-driven sectoral model results, and inverse modeling estimates, extending the 71 previous period 1990-2018 to the year 2020 to the extent possible.BU and TD products are compared with European 72 National Greenhouse Gas Inventories (NGHGIs) reported by Parties including the year 2019 under the United Nations 73 Framework Convention on Climate Change (UNFCCC).The uncertainties of the EU27+UK NGHGI were evaluated 74 using the standard deviation reported by the EU Member States following the guidelines of the Intergovernmental 75 Panel on Climate Change (IPCC) and harmonized by gap-filling procedures.Variation in estimates produced with 76 other methods, such as atmospheric inversion models (TD) or spatially disaggregated inventory datasets (BU), 77 originate from within-model uncertainty related to parameterization as well as structural differences between models.78 By comparing NGHGIs with other approaches, key sources of differences between estimates arise primarily in 79 activities.System boundaries and emission categories create differences in CO2 fossil datasets, while different land 80 use definitions for reporting emissions from Land Use, Land Use Change and Forestry (LULUCF) activities result in 81 differences for CO2 land.The latter has important consequences for atmospheric inversions, leading to inversions 82 reporting stronger sinks in vegetation and soils than are reported by the NGHGI.83 For CO2 fossil emissions, after harmonizing estimates based on common activities and selecting the most 84 recent year available for all datasets, the UNFCCC NGHGI for the EU27+UK accounts for 3392 ± 49 Tg CO2 yr -1 85 (926 ± 13 Tg C yr -1 ), while eight other BU sources report a mean value of 3340 [3238,3401] [25th,75th percentile] Tg 86 CO2 yr -1 (948 [937,961] Tg C yr -1 ).The sole top-down inversion of fossil emissions currently available accounts for 87 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 88 yet available.For the net CO2 land fluxes, during the most recent five-year period including the NGHGI estimates, 89 the NGHGI accounted for -91 ± 32 Tg C yr -1 while six other BU approaches reported a mean sink of -62 [-117,-49] 90 Tg C yr -1 and a 15-member ensemble of dynamic global vegetation models (DGVMs) reported -69 [-152,-5] Tg C yr - 91 1 .The five-year mean of three TD regional ensembles combined with one non-ensemble inversion of -73 Tg C yr -1 92 has a slightly smaller spread (0th-100th percentile of [-135,45] Tg C yr -1 ), and was calculated after removing land-93 atmosphere CO2 fluxes caused by lateral transport of carbon (crops, wood trade and inland waters) resulting in 94 increased agreement with the the NGHGI and bottom-up approaches.Results at the sub-sector level (Forestland, 95 Cropland, Grassland) show generally good agreement between the NGHGI and sub-sector-specific models, but results 96 for a DGVM are mixed.Overall, for both CO2 fossil and net CO2 land fluxes, we find current independent approaches 97 a déplacé vers le bas [1]: The work integrates updated 98 emission inventory data, process-based model results, data-99 driven sectoral model results, and inverse modeling estimates, 100 extending the previous period 1990-2018 to the year 2020 to 101 the extent possible.102 a déplacé (et inséré) [1] a déplacé vers le bas [2]: For CO2 fossil emissions, after 103 harmonizing estimates based on common activities and 104 selecting the most recent year available for all datasets, the 105 UNFCCC NGHGI for the EU27+UK accounts for 3392 ± 49 106 Tg CO2 yr -1 (926 ± 13 Tg C yr -1 ), while eight other BU 107 sources report a mean value of 3340 [3238,3401] [25th,75th 108 percentile] Tg CO2 yr -1 (948 [937,961] Tg C yr -1 ).The sole 109 top-down inversion of fossil emissions currently available 110 accounts for 3800 Tg CO2 yr -1 (1038 Tg C yr -1 ), a value close 111 to that of the NGHGI, but for which uncertainty estimates are 112 not yet available.For the net CO2 land fluxes, during the 113 most recent five-year period including the NGHGI estimates, 114 the NGHGI accounted for -91 ± 32 Tg C yr -1 while six other 115 BU approaches reported a mean sink of -62 [-117,-49] Tg C 116 yr -1 and a 15-member ensemble of dynamic global vegetation 117 models (DGVMs) reported -69 [-152,-5] Tg C yr -1 .The five-118 year mean of three TD regional ensembles combined with 119 one non-ensemble inversion of -73 Tg C yr -1 has a slightly 120 smaller spread (0th-100th percentile of [-135,45] Tg C yr -1 ), 121 and was calculated after removing land-atmosphere CO2 122 fluxes caused by lateral transport of carbon (crops, wood 123 trade and inland waters) resulting in increased agreement with 124 the the NGHGI and bottom-up approaches.Results at the sub-125 sector level (Forestland, Cropland, Grassland) show generally 126 good agreement between the NGHGI and sub-sector-specific 127 models, but results for a DGVM are mixed.Overall, for both 128 CO2 fossil and net CO2 land fluxes, we find current 129 independent approaches are consistent with the NGHGI at the 130 scale of the EU27+UK.We conclude that CO2 emissions 131 from fossil sources have decreased over the past 30 years in 132 the EU27+UK, while large uncertainties on net uptake of CO2 133 by the land surface prevent trend identification.In addition, a 134 gap on the order of 1000 Tg C yr -1 between CO2 fossil 135 emissions and net CO2 uptake by the land exists regardless of 136 the type of approach (NGHGI, TD, BU), falling well outside 137 all available estimates of uncertainties.However, 138 uncertainties in top-down approaches to estimate CO2 fossil 139 emissions remain uncharacterized and are likely substantial.140The data used to plot the figures are available at