A consistent budgeting of terrestrial carbon fluxes.

Accurate estimates of CO emissions from anthropogenic land-use change (E) and of the natural terrestrial CO sink (S) are crucial to precisely know how much CO can still be emitted to meet the goals of the Paris Agreement. In current carbon budgets, E and S stem from two model families that differ in how CO fluxes are attributed to environmental and land-use changes, making their estimates conceptually inconsistent. Here we provide consistent estimates of E and S 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 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 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.