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Abstract

The terrestrial biosphere absorbs about 20% of fossil-fuel CO2 emissions. The overall magnitude of this sink is constrained by
the difference between emissions, the rate of increase in atmospheric CO2 concentrations, and the ocean sink. However, the
land sink is actually composed of two largely counteracting fluxes that are poorly quantified: fluxes from land-use change and
CO2 uptake by terrestrial ecosystems. Dynamic global vegetation model simulations suggest that CO2 emissions from land-use
change have been substantially underestimated because processes such as tree harvesting and land clearing from shifting cultivation
have not been considered. As the overall terrestrial sink is constrained, a larger net flux as a result of land-use change
implies that terrestrial uptake of CO2 is also larger, and that terrestrial ecosystems might have greater potential to sequester
carbon in the future. Consequently, reforestation projects and efforts to avoid further deforestation could represent important
mitigation pathways, with co-benefits for biodiversity. It is unclear whether a larger land carbon sink can be reconciled with our
current understanding of terrestrial carbon cycling. Our possible underestimation of the historical residual terrestrial carbon
sink adds further uncertainty to our capacity to predict the future of terrestrial carbon uptake and losses