A consistent budgeting of terrestrial carbon fluxes

Accurate estimates of CO₂ emissions from anthropogenic land-use change (E_LUC) and of the natural terrestrial CO₂ sink (S_LAND) are crucial to precisely know how much CO₂ can still be emitted to meet the goals of the Paris Agreement. In current carbon budgets, E_LUC and S_LAND stem from two model families that differ in how CO₂ fluxes are attributed to environmental and land-use changes, making their estimates conceptually inconsistent. Here we provide consistent estimates of E_LUC and S_LAND by integrating environmental effects on land carbon into a spatially explicit bookkeeping model. We find that state-of-the-art process-based models overestimate S_LAND by 23% (min: 8%, max: 33%) in 2012-2021, as they include hypothetical sinks that in reality are lost through historical ecosystem degradation. Additionally, E_LUC increases by 14% (8%, 23%) in 2012-2021 when considering environmental effects. Altogether, we find a weaker net land sink, which makes reaching carbon neutrality even more ambitious. These results highlight that a consistent estimation of terrestrial carbon fluxes is essential to assess the progress of net-zero emission commitments and the remaining carbon budget.