Comparing national greenhouse gas budgets reported in UNFCCC inventories against atmospheric inversions
Authors: Deng Zhu , Ciais Philippe , Tzompa-Sosa Zitely A. , Saunois Marielle , Qiu Chunjing , Tan Chang , Sun Taochun , Ke Piyu , Cui Yanan , Tanaka Katsumasa , Lin Xin , Thompson Rona L. , Tian Hanqin , Yao Yuanzhi , Huang Yuanyuan , Lauerwald Ronny , Jain Atul K. , Xu Xiaoming , Bastos Ana , Sitch Stephen , Palmer Paul I. , Lauvaux Thomas , d'Aspremont Alexandre , Giron Clément , Benoit Antoine , Poulter Benjamin , Chang Jinfeng , Petrescu Ana Maria Roxana , Davis Steven J. , Liu Zhu , Grassi Giacomo , Albergel Clément , Tubiello Francesco N. , Perugini Lucia , Peters Wouter , Chevallier Frédéric 
Published: April 2022

Abstract. In support of the global stocktake of the Paris Agreement
on climate change, this study presents a comprehensive framework to process
the results of an ensemble of atmospheric inversions in order to make their
net ecosystem exchange (NEE) carbon dioxide (CO2) flux suitable for
evaluating national greenhouse gas inventories (NGHGIs) submitted by
countries to the United Nations Framework Convention on Climate Change
(UNFCCC). From inversions we also deduced anthropogenic methane (CH4)
emissions regrouped into fossil and agriculture and waste emissions, as well as
anthropogenic nitrous oxide (N2O) emissions. To compare inversion
results with national reports, we compiled a new global harmonized database
of emissions and removals from periodical UNFCCC inventories by Annex I
countries, and from sporadic and less detailed emissions reports by
non-Annex I countries, given by national communications and biennial update
reports. No gap filling was applied. The method to reconcile inversions with
inventories is applied to selected large countries covering ∼90 % of the global land carbon uptake for CO2 and top emitters of
CH4 and N2O. Our method uses results from an ensemble of global
inversions produced by the Global Carbon Project for the three greenhouse
gases, with ancillary data. We examine the role of CO2 fluxes caused by
lateral transfer processes from rivers and from trade in crop and wood
products and the role of carbon uptake in unmanaged lands, both not
accounted for by NGHGIs. Here we show that, despite a large spread across
the inversions, the median of available inversion models points to a larger
terrestrial carbon sink than inventories over temperate countries or groups
of countries of the Northern Hemisphere like Russia, Canada and the European
Union. For CH4, we find good consistency between the inversions
assimilating only data from the global in situ network and those using
satellite CH4 retrievals and a tendency for inversions to diagnose
higher CH4 emission estimates than reported by NGHGIs. In particular,
oil- and gas-extracting countries in central Asia and the Persian Gulf region
tend to systematically report lower emissions compared to those estimated by
inversions. For N2O, inversions tend to produce higher anthropogenic
emissions than inventories for tropical countries, even when attempting to
consider only managed land emissions. In the inventories of many non-Annex I
countries, this can be tentatively attributed to a lack of reporting
indirect N2O emissions from atmospheric deposition and from leaching to
rivers, to the existence of natural sources intertwined with managed
lands, or to an underestimation of N2O emission factors for direct
agricultural soil emissions. Inversions provide insights into seasonal and
interannual greenhouse gas fluxes anomalies, e.g., during extreme events
such as drought or abnormal fire episodes, whereas inventory methods are
established to estimate trends and multi-annual changes. As a much denser
sampling of atmospheric CO2 and CH4 concentrations by different
satellites coordinated into a global constellation is expected in the coming
years, the methodology proposed here to compare inversion results with
inventory reports (e.g., NGHGIs) could be applied regularly for monitoring
the effectiveness of mitigation policy and progress by countries to meet the
objective of their pledges. The dataset constructed by this study is
publicly available at https://doi.org/10.5281/zenodo.5089799 (Deng et al., 2021).