The consolidated European synthesis of CO2 emissions and removals for the European Union and United Kingdom: 1990–2020

Author:

McGrath Matthew J.ORCID,Petrescu Ana Maria Roxana,Peylin Philippe,Andrew Robbie M.ORCID,Matthews BradleyORCID,Dentener FrankORCID,Balkovič Juraj,Bastrikov Vladislav,Becker MeikeORCID,Broquet Gregoire,Ciais PhilippeORCID,Fortems-Cheiney Audrey,Ganzenmüller RaphaelORCID,Grassi Giacomo,Harris Ian,Jones Matthew,Knauer JürgenORCID,Kuhnert Matthias,Monteil GuillaumeORCID,Munassar SaqrORCID,Palmer Paul I.ORCID,Peters Glen P.ORCID,Qiu Chunjing,Schelhaas Mart-Jan,Tarasova Oksana,Vizzarri MatteoORCID,Winkler KarinaORCID,Balsamo GianpaoloORCID,Berchet AntoineORCID,Briggs Peter,Brockmann Patrick,Chevallier FrédéricORCID,Conchedda Giulia,Crippa Monica,Dellaert Stijn N. C.ORCID,Denier van der Gon Hugo A. C.ORCID,Filipek Sara,Friedlingstein PierreORCID,Fuchs Richard,Gauss Michael,Gerbig ChristophORCID,Guizzardi Diego,Günther Dirk,Houghton Richard A.ORCID,Janssens-Maenhout GreetORCID,Lauerwald RonnyORCID,Lerink BasORCID,Luijkx Ingrid T.ORCID,Moulas Géraud,Muntean Marilena,Nabuurs Gert-Jan,Paquirissamy Aurélie,Perugini LuciaORCID,Peters WouterORCID,Pilli Roberto,Pongratz JuliaORCID,Regnier Pierre,Scholze MarkoORCID,Serengil Yusuf,Smith PeteORCID,Solazzo EfisioORCID,Thompson Rona L.ORCID,Tubiello Francesco N.ORCID,Vesala Timo,Walther SophiaORCID

Abstract

Abstract. Quantification of land surface–atmosphere fluxes of carbon dioxide (CO2) 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 (including formally managed ecosystems) 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, 2021). 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 and unmanaged ecosystems. The work integrates updated emission inventory data, process-based model results, data-driven categorical 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 the European national greenhouse gas inventory (NGHGI) 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 the NGHGI 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 926 ± 13 Tg C yr−1, while eight other BU sources report a mean value of 948 [937,961] Tg C yr−1 (25th, 75th percentiles). The sole top-down inversion of fossil emissions currently available accounts for 875 Tg C in this same year, a value outside the uncertainty of both the NGHGI and bottom-up ensemble estimates and for which uncertainty estimates are not currently available. For the net CO2 land fluxes, during the most recent 5-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 5-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 percentiles of [-135,+45] Tg C yr−1), and it was calculated after removing net land–atmosphere CO2 fluxes caused by lateral transport of carbon (crop trade, wood trade, river transport, and net uptake from inland water bodies), resulting in increased agreement with the NGHGI and bottom-up approaches. Results at the category level (Forest Land, Cropland, Grassland) generally show good agreement between the NGHGI and category-specific models, but results for DGVMs are mixed. Overall, for both CO2 fossil and net CO2 land fluxes, we find that 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 land fluxes are relatively stable: positive or negative trends larger (smaller) than 0.07 (−0.61) Tg C yr−2 can be ruled out for the NGHGI. 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, in addition to known uncertainties in top-down estimates of the land fluxes. The data used to plot the figures are available at https://doi.org/10.5281/zenodo.8148461 (McGrath et al., 2023).

Funder

Horizon 2020 Framework Programme

H2020 European Research Council

Horizon 2020

Publisher

Copernicus GmbH

Subject

General Earth and Planetary Sciences

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