Biogenic Carbon (LCA) — Definition and GHG Accounting Context
A wooden beam holds a secret the steel one next to it does not: some of its carbon was in the sky a few decades ago. A growing tree pulls CO₂ out of the atmosphere and locks it into wood, and when that wood becomes a building, the carbon comes along for the ride — stored, for now, in the structure.
That atmospheric carbon, held in living and once-living matter, is what life-cycle assessment calls biogenic carbon. Biogenic carbon is carbon that biomass absorbed from the atmosphere and holds in bio-based products — accounted for separately from fossil carbon, because it came from the air and may return to it.
Biogenic carbon is carbon absorbed from the atmosphere by growing biomass and stored in bio-based products — timber, biochar, bio-plastics, paper, natural fibres. In LCA it is tracked apart from fossil carbon: uptake into the material counts as a removal (negative), its later release as an emission (positive), which EN 15804 reports as a separate GWP-biogenic figure. It is why some bio-based materials show a negative cradle-to-gate carbon — but only a lasting benefit if the carbon stays stored.
Definition — Carbon That Came From the Air
Biogenic carbon is carbon that originates in the atmosphere and is fixed into biomass through photosynthesis — then carried into any product made from that biomass. The carbon in a timber beam, a sheet of cardboard, a cotton shirt, a bio-plastic bottle, or a lump of biochar is biogenic: every atom of it was atmospheric CO₂ that a plant captured while it grew. Life-cycle assessment singles this carbon out and accounts for it separately from fossil carbon, which was locked underground for millions of years and is only now being returned to the air.
The reason for the separate treatment is the carbon cycle. Biogenic carbon moves on a short loop — absorbed as biomass grows, released when it decays, burns, or is digested, then reabsorbed by the next generation of growth. Fossil carbon does not: burning it is a one-way transfer from the deep store to the atmosphere. Because the two behave so differently in the climate system, an LCA that lumped them together would misjudge a bio-based product, and one that ignored biogenic flows entirely would miss both the carbon a product stores and the carbon it later releases.
Getting this right matters most for bio-based materials in long-lived products — the fast-growing field of mass timber and bio-based construction — where the carbon stored in the material can rival or exceed the emissions of making it. It also connects to a related but distinct idea, the treatment of biogenic CO₂ as a memo item in a greenhouse-gas inventory; this page is about the product-LCA accounting, that one about the inventory gas.
Definition at a glance
| What it is | Atmospheric carbon fixed into biomass and held in bio-based products |
|---|---|
| Comes from | Photosynthesis — a short, cyclic carbon loop |
| Found in | Timber, paper, natural fibres, bio-plastics, biochar |
| Accounted | Separately from fossil carbon; uptake −, release + |
| Reported by | EN 15804 as a distinct GWP-biogenic indicator |
| Benefit depends on | Permanence — the carbon staying stored |
Biogenic vs Fossil Carbon
The whole point of the term is the contrast with fossil carbon. Chemically the released CO₂ is identical; what differs is where the carbon came from and how quickly it cycles:
| Biogenic carbon | Fossil carbon | |
|---|---|---|
| Source | Recently atmospheric, fixed by biomass | Geological — coal, oil, gas |
| Cycle | Short — years to decades | One-way on human timescales |
| Uptake counted? | Yes — as a removal (negative) | No prior removal to count |
| In an EPD | GWP-biogenic (separate line) | GWP-fossil |
This split is exactly why EN 15804+A2 reports climate change not as one number but broken into GWP-fossil, GWP-biogenic, and GWP-luluc (land-use change): so a reader can see how much of a product’s carbon is fossil emissions and how much is the biogenic give-and-take of growth and release.
How LCA Accounts for It: The −1 / +1 Convention
The dominant modern approach tracks biogenic carbon with a simple, symmetrical rule. Every kilogram of biogenic CO₂ absorbed into a product is recorded as a removal of −1 kg CO₂e; every kilogram later released is recorded as an emission of +1 kg CO₂e. The carbon is followed in and out, rather than assumed away.
In the EN 15804 modules, the uptake (−1) is booked at the product stage, A1–A3, where the biomass is grown and made into the product — which is what pushes some materials’ cradle-to-gate figure below zero. The release (+1) is booked later: at end of life, in the C modules, if the product is burned or decays. Over a full cradle-to-grave cycle for a product that is ultimately combusted, the biogenic −1 and +1 roughly cancel, netting close to zero — the stored carbon goes back where it came from.
An older, cruder convention — sometimes called the “0/0” or “biogenic carbon neutral” approach — simply ignores biogenic flows on the assumption that uptake and release balance. The −1 / +1 method is more honest because it makes both halves visible and, crucially, shows when each happens — which is where the real climate story lives.
Biogenic Carbon in Products
The clearest place to see biogenic carbon is in bio-based construction materials, where the stored carbon can turn a cradle-to-gate footprint negative. Live figures for two timber products — note that both are per cubic metre, a volume basis, and cannot be compared with per-kilogram material data:
| Product | Cradle-to-gate carbon kg CO₂e / m³, A1–A3 (live) |
|---|---|
| Cross-laminated timber (CLT) | -660.1 |
| Sawn hardwood | -1019.3 |
Both figures are negative because the biogenic carbon stored in the wood outweighs the fossil emissions of harvesting and processing it. The same principle applies to paper, natural fibres, and bio-plastics; biochar takes it furthest, deliberately converting biomass into a stable form of carbon designed to resist release for centuries. In every case, the negative on the page is a store — carbon parked in the material — and its value to the climate turns entirely on how long it stays parked.
The Catch: Permanence and Timing
Here is the crux, and the reason biogenic carbon is so contested: a store is only a benefit while it holds. The negative cradle-to-gate figure captures the carbon going in, but not what happens to it decades later — and that end-of-life fate can hand the carbon straight back.
If a timber building is demolished and the wood is burned or sent to landfill, the end-of-life modules (C) release the stored biogenic carbon as a positive flow — and over the whole life cycle the biogenic uptake and release net to roughly zero. The upfront negative was real, but temporary. A lasting benefit requires the carbon to stay locked up: the wood reused, kept in long-lived products, or the material (like biochar) chosen precisely because its carbon resists release. This is also why a beyond-boundary recycling or energy-recovery credit (module D) is reported separately and never netted into the headline — the same discipline that governs embodied carbon and every carbon-removal accounting rule.
Two further wrinkles sharpen the debate. Timing: storing carbon now and releasing it in sixty years is not the same as never storing it — there is a real, if temporary, benefit, which dynamic and time-corrected methods try to value. Sourcing: the uptake only counts as carbon-neutral if the biomass is genuinely regrown; wood from deforestation carries a land-use-change debt that no product-stage negative can erase. The headline minus sign, in short, is the start of the analysis — not the end of it.
Common Confusions
- Reading a negative figure as a permanent win. A negative cradle-to-gate number is stored carbon, not removed-forever carbon. If the product is later burned or landfilled, the end-of-life stage releases it.
- Confusing biogenic carbon with biogenic CO₂ as an inventory memo item. This term is the product-LCA carbon store; the memo item is how biogenic CO₂ from combustion is reported in a corporate or national GHG inventory. Related, but not the same accounting.
- Treating biogenic and fossil carbon as interchangeable. The released CO₂ is chemically identical, but the origin and cycle differ — which is why EN 15804 reports GWP-biogenic and GWP-fossil separately.
- Assuming all biomass is carbon-neutral. The uptake only offsets the release if the biomass is genuinely regrown. Wood from deforestation carries a land-use-change debt.
- Netting the module D or energy-recovery credit into the total. Beyond-boundary credits are reported separately, never subtracted from the product’s in-scope figure.
Frequently Asked Questions
Biogenic carbon is carbon that was absorbed from the atmosphere by growing biomass through photosynthesis and is held in bio-based products and materials — the carbon in timber, paper, natural fibres, bio-plastics, and biochar. In life-cycle assessment it is accounted for separately from fossil carbon, because it moves on a short, cyclic loop (absorbed as biomass grows, released when it burns or decays, reabsorbed by new growth) rather than the one-way transfer of fossil carbon from underground stores to the air. Uptake into a product is recorded as a removal and its later release as an emission, which EN 15804 reports as a distinct GWP-biogenic indicator.
Because the biogenic carbon stored in the wood outweighs the fossil emissions of harvesting and processing it, at the cradle-to-gate stage. Cross-laminated timber, for example, has a live cradle-to-gate figure of about -660.1 kg CO₂e per m³, and sawn hardwood around -1019.3 kg CO₂e per m³ — both negative, and both on a per-cubic-metre basis. The minus sign represents CO₂ the trees pulled from the atmosphere, now stored in the material. It is important to read it as stored carbon, not carbon removed forever: if the timber is later burned or landfilled, the end-of-life stage releases it again, and over the full life cycle the biogenic uptake and release roughly cancel.
Both release chemically identical CO₂, but they differ in origin and cycle. Biogenic carbon was recently in the atmosphere and was fixed into biomass by photosynthesis; it moves on a short loop of years to decades, so its uptake can be counted as a removal. Fossil carbon has been locked underground for millions of years, and burning it is a one-way addition to the atmosphere with no matching uptake. Because they behave so differently in the climate system, life-cycle assessment tracks them separately, and EN 15804+A2 reports climate change split into GWP-fossil, GWP-biogenic, and GWP-luluc (land-use change) rather than as a single number.
The dominant modern method uses a symmetrical −1 / +1 convention: each kilogram of biogenic CO₂ absorbed into a product is recorded as a removal of −1 kg CO₂e, and each kilogram later released as an emission of +1 kg CO₂e. In the EN 15804 modules, the uptake is booked at the product stage (A1–A3), which can push a bio-based material’s cradle-to-gate figure below zero, and the release is booked at end of life (the C modules) if the product is burned or decays. An older “0/0” convention simply ignores biogenic flows on the assumption they balance; the −1 / +1 approach is preferred because it makes both the storage and the eventual release visible, and shows when each occurs.
Not automatically — it depends on permanence, timing, and sourcing. Over a full life cycle, a bio-based product that is eventually burned returns the carbon it stored, netting close to zero, so the storage is real but temporary. It becomes a genuine, lasting benefit only if the carbon stays locked up — the material reused, kept in long-lived products, or (like biochar) chosen because its carbon resists release. Timing matters too: storing carbon now and releasing it decades later has a real if temporary value that dynamic methods try to capture. And the uptake only offsets the release if the biomass is genuinely regrown; wood from deforestation carries a land-use-change debt that the product-stage negative cannot erase.