Biogenic Carbon in the Life Cycle of Wood Products

ADAM ROBERTSON, M.A.Sc., P.Eng.

CO-FOUNDER AND PRINCIPAL, SUSTAINATREE

Biogenic carbon in wood products involves the carbon removed from the atmosphere through photosynthesis during tree growth, which continues to be stored inside the wood products over their life span. Unlike fossil carbon, biogenic carbon belongs to a biophysical cycle that unfolds on a human timescale. Fossil carbon, on the other hand, involves a geological process that spans thousands of years.

Biogenic carbon is part of a global carbon cycle involving two-way exchanges between carbon pools in the atmosphere, biosphere and technosphere (also known as the anthroposphere).

To fully understand this cycle, one must consider all the biogenic carbon flows circulating between these systems, as described in Figure 1, with consideration for the following:

The atmosphere corresponds to the carbon contained in climate systems;
The biosphere includes the carbon pools found in forest systems above and below ground;
The technosphere includes wood harvested and processed into long-lived wood products, for example.

Figure 1. Tracking Biogenic Carbon Flows Across the Atmosphere, Biosphere and Technosphere

Source : Forestry Innovation Investment Ltd.

Physical, Spatial and Temporal System Boundaries

When quantifying and analyzing biogenic carbon flows within the forest product system, the system’s boundaries must be defined. When analyzing wood products, the system’s physical, spatial, and temporal boundaries must be defined to ensure the accurate characterization of biogenic carbon flows.

Physical boundaries define the natural and human-influenced (anthropogenic) processes included within the scope of the analysis, the spatial boundaries delineate the extent of the forest area under consideration, while temporal boundaries determine the starting point of the assessment period, for example.

The selected system boundaries can significantly influence the way in which biogenic carbon is calculated, as well as the overall study results.

Figure 2, for example, illustrates how the chosen spatial scale affects the forest carbon flow calculations associated with harvesting. Here, the curve shows the growth of a forest stand and its carbon stock evolution until harvest. The process repeats itself over time as the trees regenerate. Each time a 1-hectare forest stand (a) is harvested, a significant portion of the biogenic carbon that was sequestered during tree growth is transferred from the forest to the wood products. This analysis therefore indicates a drastic reduction in forest carbon. However, the relative significance of this reduction in forest carbon depends on the scale under consideration. The impact is much smaller if the analysis is conducted at the scale of a 1,000-hectare forest parcel (b), or a 1-million-hectare forest (c).

Figure 2 Forest Carbon Flows Over Time Across Three Distinct Spatial Scales

Source : Forestry Innovation Investment Ltd.

Zero Emissions from Land Use

Land use is the human use or management of land within a relevant boundary. Forestry is the management of forest lands to produce wood products. It differs from land use change, like deforestation, which involves converting forested land to other uses, like agriculture, urban development, etc.

The management of forest lands for the production of wood products, in which forests are replanted and continue to grow after harvesting, does not constitute deforestation, since no land use change occurs. Thus, as long as forest land is not converted to other uses, the cycle of forest growth, harvesting, and regrowth remains uninterrupted.

The continuity of this carbon cycle is considered under most standardized calculation methods involving biogenic carbon, such as the ISO 21930:20171 standard. Thus, wood from sustainably managed forests can be considered as producing no emissions associated with land use. Sustainable forest management can be demonstrated by using wood products that are responsibly sourced and certified under a sustainable forest management system, or through national and provincial reporting practices that identify forests with stable or increasing forest carbon stocks.

Biogenic Carbon Calculations in EPDs and LCA Tools

Environmental product declarations (EPDs) for wood products are required to document the biogenic carbon flows entering the product system. These may include materials from the natural environment, reused or recycled secondary materials, or secondary fuel. These inflows are characterized by a negative factor of -1 kg eq. CO2/kg CO2 in the life cycle information module where the biogenic carbon flow enters the product system (during Phase A1, Extraction of Raw Material, for example).

When biogenic carbon leaves the product system as an atmospheric emission or biobased material (as a co-product),3 the biogenic carbon flow is characterized by a positive factor of +1 kg eq. CO2/kg CO2 in the same life cycle information module. All biogenic carbon that, at any point in the life cycle, leaves the product system (as a co-product or as recovered material) for reuse, recycling or energy recovery is calculated as an outflow of biogenic carbon from the product system.

The calculation methods for biogenic carbon vary across different building life cycle assessment (LCA) tools. The accounting assumptions and calculation methods used must be understood. These factors have a significant impact on quantitative results regarding the environmental performance of wood building products and construction systems. Some tools allow users to include or exclude biogenic carbon from the analysis (e.g., TallyLCA), others account for it separately (e.g., GESTIMAT). Other tools, like the Athena Impact Estimator, automatically calculate biogenic carbon stored in wood as a negative emission (credit) when it enters the product system and add it to the global warming potential (GWP) at the end of its life. Biogenic carbon emissions during manufacturing are considered carbon neutral and therefore excluded from the GWP impact indicator.

Finally, some tools provide optional methods when calculating biogenic carbon. In the One Click LCA software, both methods yield the same total GWP result and assume a net-zero balance over the entire life cycle, since biogenic carbon removals always equal emissions. Unlike TallyLCA and the Athena Impact Estimator, the One Click LCA calculation method does not consider any permanent biogenic carbon storage in the end-of-life scenario (like permanent storage in an aerobic landfill). Each LCA tool uses different assumptions for end-of-life scenarios, which influence the associated emissions and permanent biogenic carbon storage.

Dynamic Biogenic Carbon Flow Assessment

Wood building products can store large amounts of biogenic carbon over long periods of time, even decades. This storage significantly delays, or permanently prevents, the release of biogenic GHG emissions. As the biogenic carbon is stored within the built environment, the forest regenerates and continues to absorb and store carbon over time.

Various calculation methods are available when tracking and quantifying biogenic carbon flows throughout the life cycle of wood construction products. They are designed to track biogenic removals and emissions, along with the resulting potential effects on the climate. While static analyses assume that all biogenic carbon removals and emissions occur at the start of the study period (i.e., at time zero), dynamic methods consider the potential climate impacts associated with the timing of biogenic GHG removals and emissions, CO2 removals at the start of the life cycle, and potential emissions at the end of the life cycle, for example. The vast majority of contemporary LCA studies that seek to evaluate the environmental performance of the life cycle surrounding long-lived wood products disregard the dynamic nature of biogenic carbon flows by assuming that all biogenic carbon removals and emissions occur at the start of the study period. The static approach is an oversimplification and fails to consider the potential climate benefits associated with delayed emissions.

Over the last decade, a variety of calculation methods and climate impact indicators have been developed to improve accuracy when quantifying the impacts associated with biogenic carbon removals and emissions. Conceptually, every calculation method is similar in that they attempt to estimate the absolute or relative cumulative radiative forcing over time and attributable to GHG emissions or removals that occur at a specific point in time. The primary difference in these methods involves the various assumptions and mathematical models employed to calculate the cumulative radiative forcing value.

In conclusion, wood products can store carbon over long periods of time, and this potential climate benefit can be quantified using dynamic climate impact assessment methods.

1. ISO 21930:2017 “Sustainability in buildings and civil engineering works — Core rules for environmental product declarations of construction products and services”

2. EN 15804:2012 +A1: 2013, 3.28, amended

3. Co-product examples: Bark and wood chips during manufacturing, off cuts during construction, and reclaimed wood at the end of life.

4. Translated from the Office québécois de la langue française (2019). Radiative forcing [Terminology entry]. Vitrine linguistique. https://vitrinelinguistique.oqlf.gouv.qc.ca/fichegdt/fiche/26514631/forcageradiatif https://vitrinelinguistique.oqlf.gouv.qc.ca/fiche-gdt/fiche/26514631/forcage-radiatif

Secondary fuel is defined as “fuel recovered from previous use or from waste (3.3.11), derived from a previous product system (ISO 14040:2006, 3.28) and used as an input (ISO 14040:2006, 3.21) in another product system.”2

Radiative forcing is defined as a “disruption of the Earth system’s energy balance. Radiative forcing can be natural or anthropogenic. Natural forcing is caused by the sun and volcanic eruptions, while anthropogenic forcing includes greenhouse gases, for example. ”4

To learn more about biogenic carbon calculations

Life Cycle Biogenic Carbon Accounting - A Primer for Wood Building Products and Construction Systems https://www.naturallywood.com/wp-content/uploads/FII-Life-Cycle-Biogenic-Carbon-Accounting-report-final-aug2024-1.pdf

When to Include Biogenic Carbon in an LCA https://www.woodworks.org/resources/when-to-include-biogenic-carbon-in-an-lca/

How to Include Biogenic Carbon in an LCA https://www.woodworks.org/resources/how-to-include-biogenic-carbon-in-an-lca/

Biogenic Carbon Accounting in WBLCA Tools https://www.woodworks.org/resources/biogenic-carbon-accounting-in-wblca-tools/

Assessing the Climate Change Impacts of Biogenic Carbon in Buildings: A Critical Review of Two Main Dynamic Approaches Breton, C., Blanchet, P., Amor, B., Beauregard, R., & Chang, W.-S. (2018). Assessing the Climate Change Impacts of Biogenic Carbon in Buildings: A Critical Review of Two Main Dynamic Approaches. Sustainability, 10(6), 2020. https://doi.org/10.3390/su10062020