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).
