A consistent budgeting of terrestrial carbon fluxes

In current terrestrial carbon budgeting approaches, land-use emissions are estimated by bookkeeping models, while the natural land sink is estimated by dynamic global vegetation models. These two model types include environmental and land-use changes to different extents, and thus the carbon budget is not closed. Here we suggest an approach for consistent budgeting of land-use emissions and the natural land sink by integrating the response of vegetation and soil carbon to environmental changes, derived from dynamic global vegetation models, into a spatially explicit bookkeeping model (BLUE). Our results show that land is a cumulative net source of CO₂ since 1850, which contrasts current global carbon budgets indicating a net sink. The underlying reason is both a higher estimate of land-use emissions than previously suggested as well as a smaller natural land sink: The implementation of environmental changes, in particular the beneficial effects of rising CO2 levels on biomass stocks, increases global land-use emissions over time (14% compared to earlier estimates, averaged over 2012-2021). Our natural land sink estimate calculated under transient land cover amounts to 3.0 GtC yr^-1 for 2012-2021, which is substantially lower (by 19%) compared to earlier estimates. The reason is that earlier estimates were derived under pre-industrial land cover, which ignored the large historical loss of forests (which act as a CO2 sink). Particularly in regions with strong ecosystem degradation, such as in Southeast Asia, Brazil, and Equatorial Africa, our estimates of the natural land sink substantially revise down earlier estimates. Providing a consistent estimation of terrestrial carbon fluxes is thus essential not only to provide a tangible estimate to monitor the progress of net-zero emission commitments and the remaining carbon budget, but also to highlight the need to protect remaining natural ecosystems for climate regulation.