Estimating IPCC emission factors for Agricultural and Forest Ecosystems using a multi-decade standardized eddy covariance data record (FLUXNET).

Accurate estimates of annual carbon uptake for global ecological zones, as defined by the United Nations’ Food and Agriculture Organization (FAO), are essential for preparing national greenhouse gas (GHG) inventories in the Agriculture, Forestry, and Other Land Use (AFOLU) sector. The Intergovernmental Panel on Climate Change (IPCC) 2019 guidelines emphasize the need for precise estimates of CO₂ fluxes between land and atmosphere. Traditional methods, such as the “gain-loss” approach, calculate annual carbon changes from repeated carbon stock inventories, but these methods often carry significant uncertainties, omit unobservable carbon pools, and rely on biomass-to-CO₂ conversion factors. The eddy covariance technique, developed in the late 1980s, offers a complimentary and potentially more direct measure of net ecosystem CO₂ exchange, encompassing both vegetation and soil contributions. Using data from over 400 eddy covariance sites in the global FLUXNET network, we compiled annual carbon uptake estimates across 14 FAO ecological zones and compared them to IPCC default values derived from the gain-loss method. Our analysis reveals that IPCC default inventory estimates are, on average, less than 50% of the FLUXNET-based values and are lower in 12 of 16 ecological zones. This discrepancy suggests that many GHG inventories may significantly underestimate terrestrial carbon uptake. Furthermore, the continuous and geographically diverse FLUXNET measurements capture interannual variability more effectively, providing an opportunity to refine emission factor estimates.. Incorporating FLUXNET data into national inventories could improve the accuracy and realism of GHG accounting in the AFOLU sector, though eddy covariance data is biased towards European and North American regions.