Hey Reader, have you ever read a Life Cycle Assessment?
The goal is quite straightforward: to assess all environmental impacts of a given product or process.
The final number that comes out is absolutely essential for deciding which instrument or lab item is more sustainable.
And one of the assumptions that really matters is how to account for biogenic carbon — unfortunately, there's little digestible education on the topic, but here, you’ll learn everything you need:
Today's Lesson: Explaining Biogenic Carbon
A crucial factor for all Life Cycle Assessments
Number Of The Day
To provide standardized guidelines on how to conduct Life Cycle Assessments (LCA), standards such as DIN, ISO, and PEF have been introduced. However, there is limited agreement on certain issues. Pscherer et al. showed that for biogenic carbon accounting in LCAs, there are more than 14 different standards, which sometimes differ significantly in how they handle the accounting of biogenic carbon. Today, we’ll find out why.
14
What is Biogenic Carbon?
Biogenic carbon refers to carbon that is part of natural biological cycles. It comes from organic sources like: • Plants • Animals • Microorganisms • Biomass materials (e.g., wood, paper, agricultural residues)
Plants, for example, absorb CO₂ from the atmosphere through photosynthesis. When we burn or decompose these materials, that carbon is released back into the environment.
In contrast, fossil carbon (from coal, oil, gas) comes from biomass that was locked away for millions of years. Releasing it adds "new" carbon to today’s active atmosphere.
Practical Example: Wood Product LCA
• During growth: Tree absorbs CO₂ → stores carbon in its tissues. • During use: Wooden table stores that carbon for 50 years. • End-of-life: if incinerated → CO₂ is released
The “storage” of carbon within organisms is called carbon sequestration.
Tip: Click on the picture to enlarge. This graphic stems from a paper that compared different time horizons for carbon impact calculations – in this case, for wood used in construction in Alberta. Of note, the construction sector is certainly the one with the most literature on biogenic carbon and dynamic impact assessments – unfortunately, no chance to find anything for scientific items.
In Life Cycle Assessment (LCA) and environmental impact assessments, whether carbon is biogenic or fossil changes how emissions are counted and interpreted.
Carbon Neutrality Assumption Biogenic carbon is often considered “carbon neutral”—i.e., it is equated to zero (also known as the 0/0 approach).
This is because the CO₂ released at the end of a product’s life (e.g., burning wood) is assumed to be fully balanced by the CO₂ absorbed during biomass growth.
However, there are several caveats to this approach:
1. Land-Use Change Releases Hidden Carbon • For example, when biomass harvesting causes land-use change— such as deforestation or conversion of grasslands to biomass production—carbon stored in soil and vegetation is released.
Although this graph is rather old, it demonstrates a major issue: very few actually include multiple impact assessments beyond the neutrality assumption, even though the number of publications is clearly increasing.
2. Biomass Systems Are Not Always in Balance • In real systems, harvesting biomass, processing it, and land use changes often emit additional greenhouse gases (e.g., fossil fuel use in farming, fertilizers, machinery) that are hard to estimate.
3. Growth and Regrowth Are Not Instantaneous • Time Horizon: Standard LCAs often use 100-year timeframes, but biogenic carbon storage can happen on shorter timescales—whereas tree regrowth takes time. (See our free Slack for a crazy idea how biogenic carbon might be net-negative)
This graph was published by Hoxha et al. comparing the Global Warming (GW) scores when assuming carbon uptake takes place before versus after construction/manufacturing. It outlines these scores for an analyzed building as a function of the reference service life (year 0 is the construction of the building).
4. Biogenic Carbon Can Emit Methane (CH₄) • Biomass decay under anaerobic conditions (e.g., in landfills) emits methane, which is about 34 times stronger than CO₂ in terms of global warming over 100 years.
5. Allocation and Valuation Issues • Biomass residues (waste products) and dedicated biomass (grown specifically) have different impacts—treating them the same ignores important economic and environmental differences.
Potential Solutions: An alternative to assuming carbon neutrality is the –1/+1 approach, which tracks all biogenic carbon flows throughout the product life cycle, recording both carbon uptake (–1) and carbon release (+1).
If you like to get a more detailed insight, make sure to check out Hoxha et al. since one would normally distinguish multiple modules (called process stage, use stage and end-of-life stage) in different systems (forest, building, downstream).
This method provides a more complete picture of carbon dynamics. However, it can produce misleading results if only the manufacturing phase is assessed (which some researchers recommend, since 70–80% of the global warming impact typically occurs during this phase—for example, in building construction).
Still, the –1/+1 approach is “static”—it doesn’t account for the role of time, which is crucial for biogenic carbon.
That’s why dynamic approaches were developed. There are several types: some use time-dependent characterization factors; others introduce correction factors that reflect biomass rotation periods.
Guest et al. extended these models, showing that carbon neutrality is achieved after a storage period equivalent to roughly half the biomass rotation time.
In simple terms: these approaches consider how long resources take to grow, how long products store carbon, how long they’re used, and when which side-products release emissions.
Applying The Knowledge
In essence, take every LCA with a grain of salt—there’s no perfect method. Still, in my view, we should drop the carbon neutrality assumption.
First, Paper et al. showed that the climate impact of timber construction was 6.58 kg CO₂e/m²/yr using the 0/0 approach but only 1.92 kg CO₂e/m²/yr with the –1/+1 approach.
Of course, when the end-of-life stage is included, the –1/+1 approach adds the 4.66 kg CO₂e/m²/yr back in.
Nevertheless, when adding all other impacts, the total ends up at roughly 20.66 kg CO₂e/m²/yr.
However, when applying a dynamic approach, the result jumps to 26.67 kg CO₂e/m²/yr—29% more - because wood residues from sawmills emit much of the biogenic carbon early on, and long forest rotation cycles mean only 8% of the stored CO₂ is recaptured after 100 years.
The problem with dynamic LCAs?
They’re more complex, even less standardized, and come with their own assumptions—for example, whether biomass grows before harvesting or regrows after harvesting.
This graph comes from Røyne et al., who compared the construction of a solid structure in the building envelope using different materials. Interestingly, they conducted a rather detailed analysis, including many of the metrics others leave out, demonstrating how large the differences can be depending on which factors are included. Apart from the factors you see are in/excluded, for Common vs Lowest vs Highest result GWP100, GWP500 or GWP20 were used, respectively.
As a result, depending on timeframe, emission type (CO₂ vs. CH₄), and biomass accounting method, results for the same wooden table can range from –86% to +50% climate impact.
Therefore, the best LCAs are those that model multiple scenarios and include sensitivity assessments.
Upcoming Lesson:
AI Is More Sustainable Than Human Work?
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References
Hoxha, E. et al., Biogenic carbon in buildings: a critical overview of LCA methods. 2020. Buildings and Cities 1(1), 504–524. doi:10.5334/bc.46.
Wiloso, E.I. et al., Effect of biogenic carbon inventory on the life cycle assessment of bioenergy: challenges to the neutrality assumption. 2016. J Clean Prod 125, 78–85. doi:10.1016/j.jclepro.2016.03.096.
Røyne, F. et al., Climate impact assessment in life cycle assessments of forest products: implications of method choice for results and decision-making. 2016. J Clean Prod 116, 90–99. doi:10.1016/j.jclepro.2016.01.009.
Pscherer, T. et al., LCA standards for environmental product assessments in the bioeconomy with a focus on biogenic carbon: a systematic review. 2025. Int J Life Cycle Assess 30, 371–393. doi:10.1007/s11367-024-02387-7.
Head, M. et al., Temporally-differentiated biogenic carbon accounting of wood building product life cycles. 2021. SN Appl Sci 3, 62. doi:10.1007/s42452-020-03979-2.
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