National CO₂budgets (2015–2020) inferred from atmospheric CO₂observations in support of the global stocktake

Abstract

Abstract. Accurate accounting of emissions and removals of CO2 is critical for the planning and verification of emission reduction targets in support of the Paris Agreement. Here, we present a pilot dataset of country-specific net carbon exchange (NCE; fossil plus terrestrial ecosystem fluxes) and terrestrial carbon stock changes aimed at informing countries' carbon budgets. These estimates are based on “top-down” NCE outputs from the v10 Orbiting Carbon Observatory (OCO-2) modeling intercomparison project (MIP), wherein an ensemble of inverse modeling groups conducted standardized experiments assimilating OCO-2 column-averaged dry-air mole fraction (XCO2) retrievals (ACOS v10), in situ CO2 measurements or combinations of these data. The v10 OCO-2 MIP NCE estimates are combined with “bottom-up” estimates of fossil fuel emissions and lateral carbon fluxes to estimate changes in terrestrial carbon stocks, which are impacted by anthropogenic and natural drivers. These flux and stock change estimates are reported annually (2015–2020) as both a global 1∘ × 1∘ gridded dataset and a country-level dataset and are available for download from the Committee on Earth Observation Satellites' (CEOS) website: https://doi.org/10.48588/npf6-sw92 (Byrne et al., 2022). Across the v10 OCO-2 MIP experiments, we obtain increases in the ensemble median terrestrial carbon stocks of 3.29–4.58 Pg CO2 yr−1 (0.90–1.25 Pg C yr−1). This is a result of broad increases in terrestrial carbon stocks across the northern extratropics, while the tropics generally have stock losses but with considerable regional variability and differences between v10 OCO-2 MIP experiments. We discuss the state of the science for tracking emissions and removals using top-down methods, including current limitations and future developments towards top-down monitoring and verification systems.

Carbon uptake in Eurasian boreal forests dominates the high‐latitude net ecosystem carbon budget

“…Our model‐based analysis indicates that for the 2003–2015 study period the Arctic‐boreal region, as a whole, was a NEE sink (−850 Tg CO 2 ‐C year −1 , with an associated uncertainty of ±744–1138 Tg CO 2 ‐C year −1 ). This finding is closely aligned with atmospheric budgets from the OCO‐2 MIP LNLGIS experiment (Byrne, Baker, et al, 2022; Byrne, Liu, et al, 2022), which used satellite‐retrieved column‐averaged dry‐air atmospheric mole fractions (providing finer‐spatial tracking of CO 2 ), in addition to in situ CO 2 measurements. For bottom‐up models, our results were most closely within the range of two EC tower and remote‐sensing informed machine learning approaches—FluxCom RS + METEO (Jung et al, 2020) and Virkkala et al (2021).…”

“…SMAP L4_C and CLM 5.0 indicated a small NEE source (8 and 21 Tg CO 2 ‐C year −1 ), relative to a much larger source (245 Tg CO 2 ‐C year −1 ) estimated by FluxCom RS. Whereas TCFM‐Arctic, the ACI ensemble (Liu et al, 2020), the OCO‐2 MIP experiments (Byrne, Baker, et al, 2022; Byrne, Liu, et al, 2022), FluxCom RS + METEO and Virkkala et al (2021) showed a small to moderate NEE sink (−16, −21, −49 to −32, −80, −97 Tg CO 2 ‐C year −1 , respectively).…”

“…Comparisons of total annual fluxes across the full Arctic‐boreal domain for years 2003–2015, estimated from the TCFM‐Arctic and other models: The satellite‐informed SMAP L4C flux product (Kimball et al, 2012); statistically upscaled CO 2 estimates from FluxCom (Jung et al, 2020; RS and RS + METEO) and Virkkala et al (2021); an arctic variant of the Community Land Model version 5 (CLM 5; Birch et al, 2021); results from six atmospheric CO 2 inversions (ACI; Liu et al, 2020); v10 Orbiting Carbon Observatory inversion IS, LNLG, and LNLGIS experiment results (OCO‐2 MIP; Byrne, Baker, Baker, et al, 2022; Byrne, Liu, et al, 2022); statistically upscaled CH 4 estimates from Peltola et al (2019); CarbonTracker‐CH 4 from Bruhwiler et al (2014). Annual flux budgets are shown for (a) GPP, (b) R eco , (c) NEE, and (d) CH 4 .…”

“…In contrast to the boreal zone, a majority of the models evaluated here (including TCFM‐Arctic) indicated the tundra domain as being, on average, neutral or a small source for NEE. However, adjusting the TCFM‐Arctic tundra NEE budget to account for a potentially large underestimation of episodic CO 2 emissions during spring and autumn shoulder seasons (Arndt et al, 2020; Byrne, Baker, et al, 2022; Byrne, Liu, et al, 2022; Commane et al, 2017; Liu et al, 2022; Schiferl et al, 2022) would shift tundra NEE status more toward an annual carbon source.…”

“…We compared the resulting TCFM‐Arctic budgets with regional flux estimates (SI Section 8) from an Arctic‐boreal version of the Community Land Model Version 5 (CLM 5; Birch et al, 2021); satellite‐informed SMAP L4_C and MODIS (MOD17A2H) CO 2 flux products (Kimball et al, 2012; Running et al, 2004); statistically upscaled CO 2 estimates from Virkkala et al (2021) and FluxCom (Jung et al, 2020); statistically upscaled CH 4 estimates from Peltola et al (2019), and results from atmospheric inversions—Atmospherically‐enhanced Inversion (ACI) models (Liu et al, 2020, 2022) and the v10 Orbiting Carbon Observatory‐2 inversion modeling intercomparison project (v10 OCO‐2 MIP; Byrne, Baker, et al, 2022; Byrne, Liu, et al, 2022) experiments. Additionally, we evaluated our terrestrial CH 4 emission estimates against assimilation records from CarbonTracker‐CH 4 (Bruhwiler et al, 2014).…”