On the contribution of forest bioenergy to climate change mitigation

Abstract: Greenhouse gas (GHG) emissions have to be drastically reduced to keep global warming below 2 degrees. Bioenergy can play a role in climate change mitigation by substituting for fossil fuels. However, climate benefits associated with forest-based bioenergy are being questioned, and studies arrive at contrasting conclusions, mainly due to diverging methodological choices and assumptions. This thesis combines three papers to bring together different methodological perspectives to improve the assessment and understanding of the contribution of forest bioenergy to climate change mitigation. The thesis concerns carbon balances and GHG-mediated climate effects associated with the use of forest biomass for energy in Sweden. More specifically, the focus is on methodological choices including definition of spatial and temporal system boundaries, and character of forests and forest product markets, e.g., forest owners’ responses to changes in demand for forest products, and how different assessment scales and metrics capture the difference in timing between emission and sequestration of carbon in forests that are managed with long rotations. The results show that the assessed climate benefits of promoting forest bioenergy systems can differ depending on the scale of the assessment, the forest structure, market prospects for bioenergy and other forest products, and energy system developments. Based on these findings, we recommend that assessments intending to support policy-making (i) consider how an increase in bioenergy demand affects the forest carbon stock at the landscape level, i.e., the scale at which forest operations are typically coordinated; (ii) be context-specific rather than feedstock-specific; (iii) consider changes in forest management driven by increased bioenergy demand, which can affect forest carbon stock and climate change mitigation; (iv) combine the assessment with energy system modeling to understand the size and development of bioenergy demand and different technology pathways; and (v) acknowledge short-term vs. long-term benefits, as some bioenergy systems could be associated with initial forest stock losses but great long-term benefits that can be overlooked if the temporal scope is too narrow. The latter is especially relevant when the ultimate goal is a long-term climate target, e.g.., the 2-degree target. This thesis also shows that the Swedish forest sector can make an important contribution to the 2045 goal of climate neutrality, i.e., no net GHG emissions to the atmosphere, by supplying forest fuels and other products while maintaining or enhancing carbon storage in vegetation, soils, and forest products. The results indicate that the neutrality target can only be reached by 2050 if the net carbon balance effect from the forest is considered. Additionally, measures to enhance forest productivity can increase the output of forest products (including bioenergy) and also enhance carbon sequestration in forests and products, reaching net negative emissions earlier. All in all, studies intending to support policy- and decision-making may provide more relevant information if the focus is shifted from assessing individual bioenergy systems to consider all forest products and how forest management planning as a whole is affected by bioenergy incentives - and how this in turn affects carbon balances in forest landscapes and forest product pools. Studies should preferably employ several alternative scenarios for critical factors, including policy options, forest product markets, and energy technology pathways.