Titel: Climate impact of the sustainable use of forest biomass in energy and material systems – a life cycle perspective
Fakultet: Fakulteten för teknik
Datum: Torsdagen den 18 januari 2018 kl 10.00
Plats: Sal M1083 (Södrasalen), hus M, Växjö
Opponent: Docent Göran Berndes, Chalmers tekniska högskola
Betygsnämnd: Professor Tomas Lundmark, SLU
Professor Pål Börjesson, Lunds universitet
Professor Per-Anders Hansson, SLU
Ordförande: Professor Björn Johannesson, Institutionen för byggteknik, Linnéuniversitetet
Handledare: Professor Johan Bergh, Institutionen för skog och träteknik, Linnéuniversitetet
Examinator: Professor Marie Johansson, Institutionen för byggteknik, Linnéuniversitetet
Spikning: Torsdagen den 21 december 2017 kl 14.00 på Universitetsbiblioteket i Växjö
Earth's climate is essential for our life on the planet, though the way we live is changing the climate. Human society releases greenhouse gas emissions to the atmosphere while providing housing, heat, mobility and industrial production. Man-made greenhouse gas emissions are the main causes of climate change, coming mainly from burning fossil fuels like coal, oil and fossil gas, but land-use changes also contribute significantly to total emissions. Climate scientists have observed that carbon dioxide concentrations in the atmosphere have been increasing significantly over the past century, compared to the pre-industrial era level, which cause an increased level in global surface temperature known as global warming. In order to prevent serious impacts on human life, research indicates that global warming needs to be limited to less than 2 °C and global efforts are being made in order to stay below this level.
Sustainably managed forests play an important role in the discussion on the mitigation of climate change with the prospect of sustainably providing essential materials and services as part of a low-carbon economy, both through the substitution of fossil-intensive fuels and material and through their potential to capture and store carbon in the long-term perspective.
The overall aim of this thesis and the studies was to develop a methodology under a life cycle perspective to assess the climate impact of the sustainable use of forest biomass in bioenergy and bio-based material systems. For this, the development of system analysis and scenario techniques is necessary to study how different bioenergy systems in the forest, heat, power and transportation sector could contribute to climate change mitigation. To perform this kind of analysis a methodological framework is needed to accurately compare the different biological and technological systems with the aim to minimize the net CO2 emissions to the atmosphere and hence the climate impact. In such a comparison, the complete energy supply chains from natural resources to energy end-use services has to be considered and are defined as the system boundaries.
The results show that increasing biomass production through more intensive forest management or the usage of more productive tree species combined with substitution of non-wood products and fuels can significantly reduce cumulative radiative forcing, in other words global warming. The emissions from intensified forest management resulted in very little radiative forcing in comparison to the negative radiative forcing from using the increased forest growth for biomass substitution. The biggest single factor causing radiative forcing reduction was using timber to produce wood material to replace energy-intensive construction materials such as concrete and steel. Another very significant factor was replacing fossil fuels with forest residues from forest thinning, harvest, wood processing, and post-use wood products. The fossil fuel that was replaced by forest biomass affected the reductions in greenhouse gas emissions and radiative forcing, with carbon-intensive coal being most beneficial to replace. The climate benefits of fertilization were proportional to the increased rate of biomass production, in terms of shortened rotation lengths and increased harvest volumes. Because the substitution benefits of forest product use are cumulative, and the carbon sink in the forest biomass and soil is limited, the non-management and non-use of forest biomass becomes less attractive as the time horizon increases. Over the long term, an active and sustainable management of forests, including their use as a source for wood products and biofuels, allows the greatest potential for reducing net carbon emission.
For future work it is recommended to perform this kind of analyses on the landscape level. To improve accuracy, albedo changes due to forest management and climate change should be incorporated into the framework, using a spatially specific surface albedo model as well as a model to translate changed surface albedo to climate change effects.