Large differences in land use emission quantifications implied by definition discrepancies

Abstract

The quantification of CO 2 emissions from anthropogenic land use and land use change ( e LUC) is essential to understand the drivers of the atmospheric CO 2 increase and to inform climate change mitigation policy. Reported values in synthesis reports are commonly derived from different approaches (observation-driven bookkeeping and process-modelling) but recent work has emphasized that inconsistencies between methods may imply substantial differences in e LUC estimates. However, a consistent quantification is lacking and no concise modelling protocol for the separation of primary and secondary components of e LUC has been established. Here, we review the conceptual differences of e LUC quantification methods and apply an Earth System Model to demonstrate that what is claimed to represent total e LUC differs by up to ~20% when quantified from ESM vs. offline vegetation models. Under a future business-as-usual scenario, differences tend to increase further due to slowing land conversion rates and an increasing impact of altered environmental conditions on land–atmosphere fluxes. We establish how coupled Earth System Models may be applied to separate component fluxes of e LUC arising from the replacement of potential C sinks/sources and the land use feedback and show that secondary fluxes derived from offline vegetation models are conceptually and quantitatively not identical to either, nor their sum. Therefore, we argue that synthesis studies and global carbon budget accountings should resort to the "least common denominator" of different methods, following the bookkeeping approach where only primary land use emissions are quantified under the assumption of constant environmental boundary conditions.