The consolidated European synthesis of CO2 emissions and removals for the European Union and United Kingdom: 1990–2018
Authors: Petrescu Ana Maria Roxana , McGrath Matthew J. , Andrew Robbie M. , Peylin Philippe , Peters Glen P. , Ciais Philippe , Broquet Gregoire , Tubiello Francesco N. , Gerbig Christoph , Pongratz Julia , Janssens-Maenhout Greet , Grassi Giacomo , Nabuurs Gert-Jan , Regnier Pierre , Lauerwald Ronny , Kuhnert Matthias , Balkovič Juraj , Schelhaas Mart-Jan , Denier van der
Gon Hugo A. C. , Solazzo Efisio , Qiu Chunjing , Pilli Roberto , Konovalov Igor B. , Houghton Richard A. , Günther Dirk , Perugini Lucia , Crippa Monica , Ganzenmüller Raphael , Luijkx Ingrid T. , Smith Pete , Munassar Saqr , Thompson Rona L. , Conchedda Giulia , Monteil Guillaume , Scholze Marko , Karstens Ute , Brockmann Patrick , Dolman Albertus Johannes 
Published: May 2021

Abstract. Reliable quantification of the sources and sinks of atmospheric carbon
dioxide (CO2), including that of their trends and uncertainties, is
essential to monitoring the progress in mitigating anthropogenic emissions
under the Kyoto Protocol and the Paris Agreement. This study provides a
consolidated synthesis of estimates for all anthropogenic and natural
sources and sinks of CO2 for the European Union and UK (EU27 + UK),
derived from a combination of state-of-the-art bottom-up (BU) and top-down
(TD) data sources and models. Given the wide scope of the work and the
variety of datasets involved, this study focuses on identifying essential
questions which need to be answered to properly understand the differences
between various datasets, in particular with regards to the less-well-characterized fluxes from managed ecosystems. The work integrates recent
emission inventory data, process-based ecosystem model results, data-driven
sector model results and inverse modeling estimates over the period
1990–2018. BU and TD products are compared with European national greenhouse gas
inventories (NGHGIs) reported under the UNFCCC in 2019, aiming to assess and
understand the differences between approaches. For the uncertainties in
NGHGIs, we used the standard deviation obtained by varying parameters of
inventory calculations, reported by the member states following the IPCC
Guidelines. Variation in estimates produced with other methods, like
atmospheric inversion models (TD) or spatially disaggregated inventory
datasets (BU), arises from diverse sources including within-model uncertainty
related to parameterization as well as structural differences between
models. In comparing NGHGIs with other approaches, a key source of
uncertainty is that related to different system boundaries and emission
categories (CO2 fossil) and the use of different land use definitions
for reporting emissions from land use, land use change and forestry (LULUCF)
activities (CO2 land). At the EU27 + UK level, the NGHGI (2019)
fossil CO2 emissions (including cement production) account for 2624 Tg CO2 in 2014 while all the other seven bottom-up sources are consistent
with the NGHGIs and report a mean of 2588 (± 463 Tg CO2). The
inversion reports 2700 Tg CO2 (± 480 Tg CO2), which is well in line
with the national inventories. Over 2011–2015, the CO2 land
sources and sinks from NGHGI estimates report −90 Tg C yr−1 ±  30 Tg C yr−1
while all other BU approaches report a mean sink of −98 Tg C yr−1 (± 362 Tg of C from dynamic global vegetation
models only). For the TD model ensemble results, we
observe a much larger spread for regional inversions (i.e., mean of 253 Tg C yr−1 ± 400 Tg C yr−1). This concludes that (a) current
independent approaches are consistent with NGHGIs and (b) their uncertainty is too
large to allow a verification because of model differences and probably
also because of the definition of “CO2 flux” obtained from different
approaches. The referenced datasets related to figures are visualized at
https://doi.org/10.5281/zenodo.4626578 (Petrescu et al., 2020a).