A Global, Daily Carbon Budget for Terrestrial Ecosystems Constrained by Satellite Observations of Soil Moisture: The SMAP Level 4 Carbon Product at Ten Years

The capacity of terrestrial ecosystems to retain carbon or sequester more atmospheric carbon is frequently investigated as a potential natural climate solution. However, global carbon inventories, national carbon assessments, and atmospheric inversion studies suffer from key limitations: infrequent estimates, low spatial resolution, or a lack of partitioning into different carbon sinks and sources. In contrast, process-based ecosystem models can represent ecosystem biogeochemical fluxes on daily time scales, globally, constrained by near-real time observations from satellite sensors. The NASA Soil Moisture Active Passive mission Level 4 Carbon (L4_C) data set now provides over a decade of daily, global estimates of gross primary production (GPP), soil heterotrophic respiration, net ecosystem exchange, and soil organic carbon for terrestrial ecosystems. We present a validation of the latest L4_C product release, Version 8 (V8), and its updates compared to the previous version. Since 2015, L4_C has consistently met its performance target and provides carbon flux estimates similar to or better than global extrapolations such as FLUXCOM-X when compared to eddy-covariance tower measurements (ubRMSE of NEE (Formula presented.), GPP = 1.13 g C (Formula presented.)). We also compare L4_C V8 to the magnitude and direction of trends in global carbon uptake and net carbon balance predicted by the TRENDYv13 ensemble of dynamic global vegetation models. Based on regional case studies for recent extremes of fire weather, heat, and drought, L4_C demonstrates sensitivity to the impacts of short-term climate variability on the terrestrial carbon cycle and provides a basis for near-real time assessment of global carbon sinks and sources.