Uncertainties in fossil fuel emissions affects both reporting and verification as well as has large implications for carbon budget analysis

The transparent monitoring, planning, and verification of carbon neutrality targets are increasingly relying on observation-based accounting of carbon emissions and removals to support the Paris Agreement. The top-down inversion method, widely used to estimate natural carbon fluxes from observed CO₂ levels, hinges on near-perfect knowledge of fossil fuel CO₂ (FFCO₂) emissions. However, the latest GridFEDv2023.1 inventory highlights substantial uncertainties in FFCO₂ emission data, particularly for the 2000s—a crucial period for many countries establishing baselines for Nationally Determined Contributions (NDCs) and net-zero emission targets. This study, employing the MIROC4-based inversion system with the FFCO2 emission uncertainties, strongly suggests that substantial increase rate in the inversion estimated global land-biospheric uptake (~0.22 PgC yr^-2) from 2001 to 2011 is likely a result of overestimated FFCO₂ emission rates, using the commonly accepted FFCO2 emission. The major source of uncertainty in the increase rate of FFCO2 emissions arises from China, while large systematic over/under-estimations are likely for the coterminous USA and western Europe. The lower bound of GridFED FFCO₂ growth rate in China brings the inversion estimated global and China land-biospheric carbon sink increase in line with TRENDY model fluxes (~0.16 and 0.001 PgC yr^-2, respectively).  The flux estimations for the US and Europe also suffer from the uncertainty in the FFCO2 emission assumption - lower and higher FFCO2 emissions are recommended based on the comparison of inversion and TRENDY results, while uncertainties in model transport and observational data coverage are ignored.