Assessing global progress towards achieving the Paris Agreement requires rigorous measurement of aggregate national actions and commitments based on modeled mitigation pathways. However, National Greenhouse Gas Inventories (NGHGI) and scientific assessments of anthropogenic emissions follow different accounting conventions for terrestrial carbon fluxes, resulting in large differences in current emission estimates, a gap that will evolve over time. Using state-of-the-art methodologies and a ground-based carbon cycle simulator, we align the mitigation measures assessed by the Intergovernmental Panel on Climate Change (IPCC) with the NGHGI to make a comparison. We find that key global mitigation benchmarks become more difficult to achieve when calculated using NGHGI conventions, requiring an earlier zero-net-CO timing ₂ and lower cumulative emissions. Furthermore, the attenuation of natural carbon removal processes, such as carbon fertilization, may mask anthropogenic soil removal efforts, with the result that terrestrial carbon fluxes in the NGHGI may ultimately become sources of emissions by 2100. Our results are important to the Global Review, suggesting that nations will need to increase the collective ambition of their climate goals to stay in line with global temperature targets. (Matthew J. Gidden , Thomas Gasser, Keywan Riahi )