Differences and uncertainties in land-use CO2 flux estimates, and how to overcome them

Accurate estimates of land-use change CO₂ fluxes (F_LUC) are essential for understanding the terrestrial carbon cycle and for informing national and global climate reporting. F_LUC can be quantified using a range of approaches, including bookkeeping models, dynamic global vegetation models (DGVMs), national greenhouse gas inventories, Earth observation-based products, and atmospheric inversions. However, systematic comparison of F_LUC estimates across approaches remains challenging due to fundamental methodological differences. Additionally, individual approaches are often associated with substantial uncertainties.

Here, we provide an overview of the main sources of uncertainty and methodological discrepancies across approaches, while highlighting recent advances and identifying promising directions to further harmonize and improve F_LUC estimates. Key sources of uncertainty and inconsistency include differing objectives and definitions across approaches, differences in the separation of natural and anthropogenic drivers (leading to F_LUC differences of up to 2.8 PgC yr^-1), incomplete process representation (contributing up to 30% uncertainty in F_LUC), uncertainties in land-use data (up to 30% uncertainty), and constraints related to spatial resolution. At the same time, several important methodological improvements have been achieved recently, including the consideration of environmental changes in bookkeeping-based F_LUC estimates and the correction for replaced sinks and sources (RSS) in DGVM-based F_LUC estimates. Ongoing research projects are addressing several remaining challenges, including the improvement of regrowth estimates, the quantification of disturbance impacts, and the consolidation and harmonization of different land-use datasets. These developments build primarily on combining high-resolution Earth observation-based databases with the flexibility and traceability of semi-empirical modelling. Together, these advances provide an important contribution towards more consistent and robust estimates of land-use change CO₂ fluxes within and across approaches.