Mass Timber Rising
Tall Wood – Low Carbon
Fuels to Frame
A Future Built of Wood
Strategies to combat climate change often focus on reducing fossil energy use and switching to renewable energy. Renewable energy has made great strides and that progress needs to continue. However, as renewable energy advances and the grid decarbonizes, our attention turns to how we build and what we build with. Reducing carbon dioxide in the atmosphere from the built environment requires a two pronged strategy: greater use of renewable energy and widespread adoption of renewable low-carbon building materials, such as mass timber.
Wood dominates residential construction in the U.S., but taller buildings are mainly concrete and steel which contain much higher embodied carbon. Switching to mass timber harvested from sustainable forests avoids the emissions from the manufacture of concrete and steel, a process known as substitution. Substitution permanently avoids the emissions associated with fossil-intensive materials. Researchers have shown using mass timber from actively managed sustainable forests to displace concrete and steel results in substantially lower carbon emissions than unmanaged forests and buildings constructed of conventional materials.1
This animation is modeled on new peer-reviewed research that examines carbon levels of an unmanaged forest and sustainably-harvested actively managed forest. It takes those numbers and combines them with peer-reviewed carbon impacts derived from apples-to-apples life cycle assessments (LCA).
The scenario on the left shows a high quantity of carbon stored in a mature unmanaged forest. Terrestrial carbon, shown in green, is flat because the forest has reached a point where growth, decay, and disturbances have equalized. The new buildings are built with conventional materials of concrete and steel. Each new building adds CO2 to the atmosphere due to their high embodied carbon. Atmospheric carbon from the buildings remains in the atmosphere and accumulates over time.
The scenario on the right shows a managed forest with 45-year rotations and identical buildings constructed from mass timber. The terrestrial carbon accumulation is split between forest carbon (green) and wood product carbon (brown). The conventional materials scenario shows no accumulation of wood products carbon but the mass timber scenario shows a stable pool of wood products carbon as buildings are built and then retired. Manufacturing emissions from the production of wood products accumulate in the atmosphere but at a much lower rate.
This real-world comparison illustrates how mass timber from sustainably-managed forests results in lower carbon emissions from the production of similar buildings.
A recent apples-to-apples analysis in peer-reviewed research showed that an eight-story mass timber building built in the Northwest would contribute 43% less emissions than a comparable concrete building built in the same location. The reduction is possible because wood products from sustainable forests are considered carbon-neutral as long as forest growth exceeds harvest and decay levels, which is the case in Northwest forests. By building with biogenic carbon as opposed to fossil carbon the emissions are almost cut in half.2
The comparison presents two simplified versions of forest management to convey a point. In reality, forest management is highly complex.
Undisturbed forests do not have infinite sequestration capacity. Over time, forests reach a dynamic equilibrium where growth equals mortality and they move from sequestering to storing. And, depending on circumstances, may become a source of emissions. They basically fill up, like a sponge, until ecological pressures force carbon release.
Researchers at University of Washington4 explain that,
“Simply growing a forest provides a net one-time removal of CO2 from the atmosphere, not a sustainable process for removing more CO2 for centuries. To be a constant source of carbon reduction trees need to be harvested, store carbon, and displace fossil fuel-intensive products.”
Only considering standing forests discounts the crucial role of forest products as a substitute for fossil fuel-intensive building materials. Wood products are grown via photosynthesis and manufactured in facilities that use a high proportion of carbon-neutral biomass energy. This results in wood products having very low embodied carbon. Mass timber can substitute for fossil fuel intensive-products such as walls and floors which avoids the CO2 emissions associated with their production. For example, the carbon stored in wood studs is nearly twice that of steel studs (carbon storage plus avoided emissions).5
The foundation of using forest products to reduce fossil carbon emissions is based on forest sustainability. Specifically, the forest biomass must be equal to or greater than the amount harvested and lost to natural death and decay. This is the assumption of “constant forest carbon stocks”. The forests of Oregon and Washington easily meet these baseline requirements. The Northwest has both an active forest products industry and expanding forest carbon stocks. Oregon forest carbon stocks have increased by 11% from 1990 to 2019 and Washington forest carbon stocks grew by six percent during the same period.6
Decarbonizing the built environment is a grand challenge. The region has already made significant gains in renewable energy and forest sustainability. For sustained progress, we’ll need to put our forests to work storing more carbon and displacing fossil-intensive building materials. It’s time to build a future made of wood.
2 Puettmann, Pierobon, Ganguly, Gu, Chen, Liang, Jones, Maples, and Wishnie. 2021?. Comparative LCAs of conventional and mass timber buildings in regions with potential for mass timber penetration. Sustainability special issue – Mass Timber and Sustainable Building Construction. In review.
3 Gray, A. N., T. R. Whittier, and M. E. Harmon. 2016. Carbon stocks and accumulation rates in Pacific Northwest forests: role of stand age, plant community, and productivity. Ecosphere 7(1):e01224. 10.1002/ecs2.1224
4 Lippke, Puettmann, Oneil and Oliver (2021) The Plant a Trillion Trees Campaign to Reduce Global Warming – Fleshing Out the Concept, Journal of Sustainable Forestry, 40:1, 1-31, DOI: 10.1080/10549811.2021.1894951
5 Lippke, Puettmann, and Oneil. (2019, December). Effective Uses of Forest-Derived Products to Reduce Carbon Emissions. Consortium for Research on Renewable Industrial Material (CORRIM) Technical Note 1. https://corrim.org/use-of-forest-products-to-reduce-carbon-emissions
6 Forest Carbon Report: Oregon. (2020). Forest Resources Association. https://forestresources.org/policy-priorities/carbon-fact-sheets
Forest Carbon Report: Washington. (2020). Forest Resources Association. https://forestresources.org/policy-priorities/carbon-fact-sheets