Reconciling top-down and bottom-up modelling on future bioenergy deployment

1 There is a divergence of views on future bioenergy deployment that is based in disparate epistemic communities. Integrated assessment models (IAMs) project rising deployment of biomass and biofuels in climate change-mitigation scenarios1,2. In contrast, life-cycle assessments (LCAs) and partial equilibrium models of land-use change emphasize high up-front greenhouse-gas emissions from direct land-use change (LUC)3,4 and indirect land-use change (ILUC)5, and highlight epistemic uncertainties in modelling greenhouse-gas emissions as exemplified in fat-tail distributions and associated high risks6. Furthermore, bioenergy deployment is regarded as a threat to carbon-rich natural land, biodiversity, water resources and food security7. The Intergovernmental Panel on Climate Change (IPCC)’s Special Report on Renewable Energy Sources and Climate Change Mitigation (SRREN)8 exemplifies the seemingly disparate findings across these communities, highlighting the risk of land-use change and other trade-offs in its chapters on bioenergy9 and sustainable development10, without integrating these results in its assessment chapter on mitigation potential and costs11. This lack of reconciliation constrains the assessment process and highlights the need for a coordinated research agenda. We briefly review LCA studies that indicate potentially high but uncertain life-cycle emissions, and highlight that assessments of biofuel emissions often use mixed and inadequate methodologies. We show that IAMs heavily rely on bioenergy to achieve future climate change-mitigation targets. Highly variable modelling assumptions of IAMs allow for widely diverging results. IAMs also focus on first-best world scenarios, that is, they specify assumptions of quasi-perfect worlds, and thus systematically underexplore risks related to ILUC and nitrous oxide emissions in imperfect real-world situations. We provide an outlook of how a modular modelling framework, integrating inductive bottomup and deductive top-down perspectives, can fill this gap. We argue that improved interdisciplinary communication is necessary to achieve this. We conclude by exploring the implications of a more complete representation of uncertainties at the science/ policy interface. Reconciling top-down and bottom-up modelling on future bioenergy deployment