The terrestrial biosphere absorbs about 20% of fossil-fuel CO2 emissions. However, the land sink is 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 and that terrestrial ecosystems might have greater potential to sequester carbon in the future.