Seasonal Characteristics of Model Uncertainties From Biogenic Fluxes, Transport, and Large-Scale Boundary Inflow in Atmospheric CO₂ Simulations Over North America

Regional estimates of biogenic carbon fluxes over North America from both atmospheric inversions (“top-down” approach) and terrestrial biosphere models (“bottom-up”) remain highly uncertain. We merge these approaches with an ensemble-based, regional modeling system able to diagnose and quantify the causes of uncertainties in top-down atmospheric estimates of the terrestrial sink over North America. Our ensemble approach quantifies and partitions the uncertainty stemming from atmospheric transport, the biosphere, and large-scale CO₂ boundary inflow (boundary conditions). We use meteorological data, CO₂ fluxes, and CO₂ mole fraction measurements to assure the reliability of the ensemble system. Our results show that all uncertainty components have clear seasonal variations. The biogenic flux component dominates modeled boundary layer CO₂ uncertainty, ranging from 2.5 ppm in summer and winter to 1.5 ppm in fall and spring. Spatially, it remains highly uncertain in the U.S. Corn Belt regions. Transport uncertainty reaches a maximum of 2.5 ppm in the summer months and stays at 1.2 ppm for the rest of the year and is highly correlated with the biogenic CO₂ fluxes. Boundary conditions play the smallest role in atmospheric boundary layer CO₂ uncertainty with a magnitude smaller than 1 ppm. However, boundary conditions are the most important uncertainty component in column-averaged CO₂ (XCO₂). The spatiotemporal variations of the uncertainties in modeled XCO₂ are similar to those in atmospheric boundary layer CO₂.