Is energy from woody biomass positive for the climate? – new IEA Bioenergy brief
Overall, energy from woody biomass can be very positive for the climate, particularly when applying sustainable forest management practices, and when the biomass is used efficiently such as in combined heat and power (CHP) plants and biorefineries according to a new brief from the International Energy Agency (IEA) Bioenergy. The brief provides a background and a number of concise arguments on the timely question if energy from woody biomass is positive for the climate.
The IEA Bioenergy brief ‘Is energy from woody biomass positive for the climate?’ highlights that sustainable forest management (SFM) is key to maintaining healthy and productive forests, and for controlling harvest levels so as to maintain or increase carbon stocks in forests. Forest wood, fibre and energy products derived from sustainably managed forest replace carbon-intensive materials and fossil fuels, which is “crucial in future decarbonisation strategies.”
In terms of climate impacts, the important issues are related to how the forest carbon cycle is affected by management changes to provide biomass for bioenergy in addition to other forest products. The key issue according to the IEA Bioenergy brief is the carbon sink strength of the forest; that is the net assimilation of carbon and the associated changes in carbon stock in forest soils and vegetation and/or harvested wood products, and carbon losses through natural disturbances such as fires or insect attacks.
Silvicultural operations and harvest activities including biomass extraction for energy are interacting factors influencing the development of forest carbon stocks. In a sustainably managed forest, these are coordinated across the forest landscape to maintain a healthy forest and to obtain a continuous flow of wood for society, while maintaining or increasing wood volume in the forest.
On the issue of emission timing the authors point out that when considered across the whole forest estate, stand-level fluctuations in carbon stock are evened out. If the annual cut is equal to the annual growth, at estate level, the carbon stock of the whole forest will remain constant. If the annual cut is less than the annual growth, the forest will have a net sequestration of carbon, while also providing wood for products and biomass for energy.
The authors also note that if a forest is converted to a new management regime where more residues are extracted or rotation length is reduced, the carbon stock of the forest estate may decrease, and this should be included as an emission of the bioenergy system whereas the converse could also be true, that is enhanced management stimulated by the demand for bioenergy could reduce or even negate any decline in carbon stock under the bioenergy scenario.
The authors explain that while it is “perfectly true” that more carbon dioxide (CO2) is released per unit energy from biomass than from black coal, a consequence of the chemical composition of biomass and coal respectively, there is a fundamental difference between the two carbon cycles; burning fossil fuels releases carbon in the “slow domain” that has been locked up in the ground for millions of years, while burning biomass emits carbon that is part of the biogenic carbon cycle or “fast domain”.
Thus the use of fossil fuels increases the total amount of carbon in the biosphere-atmosphere system while bioenergy systems operate within this system. Furthermore, most woody biomass sourced for energy is a by-product or residue of forestry operations and forest industry and using by-products and residues for energy have typically been found to achieve climate change mitigation benefits in the short term.
Though bioenergy can be carbon neutral, the authors point out that all emissions along the full bioenergy supply chain need to be considered as it will typically include usage of fossil energy for instance in transport. Nonetheless, the fossil energy used in the supply chain is generally a small fraction of the energy content of the bioenergy product, even for woody biomass transported over long distance
The authors also caution that the focus on short-term carbon balances may be misleading given the long-term “resilience” of CO2 in the atmosphere. Instead what matters most is whether increasing use of forest biomass for energy leads to systematic changes in the forest carbon stocks and a reduction of fossil energy use.