Regional Inversion Shows Promise in Capturing Extreme-Event-Driven CO₂ Flux Anomalies but Is Limited by Atmospheric CO₂ Observational Coverage

Extreme climate events are becoming more frequent, with poorly understood implications for carbon sequestration by terrestrial ecosystems. A better understanding will critically depend on accurate and precise quantification of ecosystems responses to these events. Taking the 2019 US Midwest floods as a case study, we investigate current capabilities for tracking regional flux anomalies with “top-down” inversion analyses that assimilate atmospheric CO₂ observations. For this analysis, we develop a regionally nested version of the NASA Carbon Monitoring System-Flux system for North America (CMS-Flux-NA) that allows high resolution atmospheric transport (0.5° × 0.625°). Relative to a 2018 baseline, we find the 2019 US Midwest growing season net carbon uptake is reduced by 11–57 TgC (3%–16%, range across assimilated CO₂ data sets). These estimates are found to be consistent with independent “bottom-up” estimates of carbon uptake based on vegetation remote sensing (15–78 TgC). We then investigate current limitations in tracking regional carbon budgets using “top-down” methods. In a set of observing system simulation experiments, we show that the ability of atmospheric CO₂ inversions to capture regional carbon flux anomalies is still limited by observational coverage gaps for both in situ and satellite observations. Future space-based missions that allow for daily observational coverage across North America would largely mitigate these observational gaps, allowing for improved top-down estimates of ecosystem responses to extreme climate events.