Significant Influence of Lateral Carbon Fluxes on Regional U.S. Carbon Budgets

Observations of atmospheric carbon dioxide (CO2) are expected to play a critical role in monitoring, reporting, and verification (MRV) designed to track progress toward the reduction goals of greenhouse gas emissions. In this study, we construct a comprehensive regional carbon budget for seven National Climate Assessment regions of the contiguous United States (CONUS) over 2015-2020. We demonstrate that lateral carbon movement across regions, along with their resulting endpoint CO2 emissions, represent a major contributor to the interpretation of regional net surface-atmosphere fluxes of CO2. When accounting for lateral carbon fluxes, “top-down” flux estimates from v10 Orbiting Carbon Observatory 2 (OCO-2) Modeling Intercomparison Project (MIP) agree, within uncertainty, with inventory-based carbon stock change estimates in six out of seven National Climate Assessment regions - except for the Southwest. The total net carbon exchange from the top down is 925±339 teragrams carbon per year (TgC/year), consistent with the 974 TgC/year estimate from the inventory. Notably, lateral carbon flows in the form of harvested wood products and riverine carbon burial are responsible for an estimated 16% of the total carbon stock changes occurring across CONUS. Averaged over the seven regions, accounting for lateral transport and their endpoint carbon emissions improving the consistency between inventory and top-down estimates by 31%. Our study presents a roadmap for reconciling the inventories and top-down atmospheric inversions at sub-national scale that highlights the importance of accurate representation of lateral carbon flows for using top-down estimates with national inventories.