Evaluation of CarbonTracker's Inverse Estimates of North American Net Ecosystem Exchange of CO₂ From Different Observing Systems Using ACT‐America Airborne Observations

Abstract

Quantification of regional terrestrial carbon dioxide (CO2) fluxes is critical to our understanding of the carbon cycle. We evaluate inverse estimates of net ecosystem exchange (NEE) of CO2 fluxes in temperate North America, and their sensitivity to the observational data used to drive the inversions. Specifically, we consider the state‐of‐the‐science CarbonTracker global inversion system, which assimilates (a) in situ measurements (IS), (b) the Orbiting Carbon Observatory‐2 (OCO‐2) v9 column CO2 (XCO2) retrievals over land (LNLG), (c) OCO‐2 v9 XCO2 retrievals ocean‐glint (OG), and (d) a combination of all these observational constraints (LNLGOGIS). We use independent CO2 observations from the Atmospheric Carbon and Transport (ACT)—America aircraft mission to evaluate the inversions. We diagnose errors in the flux estimates using the differences between modeled and observed biogenic CO2 mole fractions, influence functions from a Lagrangian transport model, Bayesian inference, and root‐mean‐square error (RMSE) and bias metrics. The IS fluxes have the smallest RMSE among the four products, followed by LNLG. Both IS and LNLG outperform the OG and LNLGOGIS inversions with regard to RMSE. Regional errors do not differ markedly across the four sets of posterior fluxes. The CarbonTracker inversions appear to overestimate the seasonal cycle of NEE in the Midwest and Western Canada, and overestimate dormant season NEE across the Central and Eastern US. The CarbonTracker inversions may overestimate annual NEE in the Central and Eastern US. The success of the LNLG inversion with respect to independent observations bodes well for satellite‐based inversions in regions with more limited in situ observing networks.

Covariation of Airborne Biogenic Tracers (CO2, COS, and CO) Supports Stronger Than Expected Growing Season Photosynthetic Uptake in the Southeastern US

“…Our primary hypothesis is that model GPP is too weak in the south, as indicated in parallel studies of ACT-America data (Cui et al, 2021;Feng et al, 2021), and plant CO 2 /COS uptake sensitivity experiments in this study. Parallel studies of the CASA biosphere model attribute low uptake to missing agricultural sinks from management that leads to biomass removal in late summer (Feng et al, 2021).…”

Evaluating Global Atmospheric Inversions of Terrestrial Net Ecosystem Exchange CO2 Over North America on Seasonal and Sub‐Continental Scales

“…The model-data comparison framework used here follows the methods presented in Cui et al (2021). This description focuses primarily on modifications to Cui et al (2021)'s methods.…”

“…The flights were designed to sample ecosystem fluxes in the Central and Eastern US and maximize the influence of NEE on ABL CO 2 . Numerical estimates in Cui et al (2021) show that the fire and ocean fluxes have negligible contributions to the ABL mole fractions. Fossil fuel sources have a more significant, but moderate impact.…”

Informing Nature‐based Climate Solutions for the United States with the best‐available science

“…Here, multi‐scale benchmarking approaches originating from earth system modelling can provide a useful path forward. For example, the International Land Model Benchmarking (ILAMB) package (Collier et al, 2018) was originally designed to evaluate and benchmark land model results through comparison with site‐, regional‐, and global‐scale observations, including from airborne CO 2 concentration measurements (Cui et al, 2021), satellite‐based remote sensing (Eldering et al, 2017), machine‐learning flux upscaling products (Jung et al, 2019), and carbon cycle models and data assimilation products (e.g., NOAA CarbonTracker, Peters et al, 2007). When referenced to CODAS ground‐truthing data, benchmarking systems like ILAMB could provide an important perspective on the magnitude and uncertainty of realized NbCS project benefits, as estimated from a range of ground‐, airborne‐, and spaceborne observations.…”