How well do we know the flux of CO₂ from land-use change?

Five new estimates of global net annual emissions of carbon from land use and land-use change collectively describe a gradually increasing trend in emissions, from ∼0.6 PgC yr⁻¹ in 1850 to ∼1.3 PgC yr⁻¹ in the period 1950–2005, with an annual range that varies between ±0.2 and ±0.4 PgC yr⁻¹ of the mean. All estimates agree in the upward trend from 1850 to ∼1950 but not thereafter. In recent decades, when rates of land-use change and biomass density should be better known than in the past, the estimates are more variable. Most analyses have used three quasi-independent estimates of land-use change that are based on national and international agricultural and forestry data of limited accuracy in many countries. Further, the estimates of biomass used in the analyses have a common but limited literature base, which fails to address the spatial variability of biomass density within ecosystems. In contrast to the sources of information that have been used to date, a combination of existing ground and remote sensing data are available to determine with far higher accuracy rates of land-use change, aboveground biomass density, and, hence, the net flux of carbon from land use and land-use change.