Biogenic carbon, stored carbon and the climate case for residues

Carbon stored in wood

Wood is about half carbon by oven-dry mass; one 2024 study uses 50%. Carbon is converted to CO2 by multiplying by 44/12, about 3.67. The declarations agree with this: 218 kg C per m3 in one Swedish declaration, 211 in the Swedish Wood declaration, 206 in another and 214.3 in a British Columbia average. Check: 218 x 3.667 = 799 kg CO2, matching the declared -800; 211 x 3.667 = 774, matching -773.

Worked example: from density to CO2

The Swedish Wood product weighs 489 kg per m3 at 16% moisture (water mass over oven-dry mass).

1. Oven-dry mass: 489 / 1.16 = 421.6 kg.
2. Carbon at 50%: 421.6 x 0.5 = 210.8 kg C.
3. CO2: 210.8 x 44/12 = 773 kg CO2.

The result reproduces the declared biogenic value.

How standards handle it

• EN 15804+A2 (Europe): uptake is counted as negative in A1 and release at end of life (module C) as positive. In a cross-laminated timber example in the portal's source, -762 in A1 is offset by +762 in C3. GWP-GHG is the same sum with biogenic CO2 set to zero.
• North American style (ISO 21930): the AWC/CWC declaration assumes all stored carbon is released at end of life, so it claims no net storage; net biogenic emissions across A1-A3 are zero.
• A British Columbia mill example shows biogenic flows of -1,520.04 in A1 and +1,561.91 in A3 beside a 45.86 kg CO2 eq total; stored carbon is not counted as long-term storage.

Storage in wood products

National inventories count storage in the harvested wood products pool using half-lives. The Swiss inventory documentation (2026) uses default half-lives of 35 years for sawnwood, 25 years for wood panels and 2 years for paper (IPCC 2014 Tier 2 defaults). A half-life is the time for half of the carbon to leave the pool. If the same rate continues, after two half-lives a quarter remains.

Example: 1,000 kg CO2 stored in panels with a 25-year half-life: after 50 years 1,000 x 0.5^2 = 250 kg remains; after 105 years in sawnwood (3 half-lives of 35 years) 0.5^3 = 12.5% remains.

Two caveats from the source. Stored carbon per m3 (roughly 700-800 kg CO2) is large compared with manufacturing fossil emissions (roughly 30-80 kg CO2 eq), but that comparison is the portal's own, across different sources and boundaries. And the benefit depends on the product staying in use, the forest regrowing and what the wood replaces. A Canadian study reports an average substitution factor of about 0.80 tC/tC for sawnwood in construction against steel and concrete (excluding biogenic carbon) and advises factors should fall as those materials decarbonise. A mill's declared GWP should not claim substitution benefits unless the EPD includes module D.

The climate case for residues

IEA Bioenergy states that forest industry by-products such as sawdust, bark and black liquor are typically associated with short-term climate benefits, while slowly decaying residues and long-rotation roundwood are associated with long payback periods. Payback is described as nearly immediate for residues used efficiently, for example in CHP plants and biorefineries. The source gives no numbers. Three points follow, partly from the source and partly from the portal's reasoning:

1. The counterfactual matters. A residue that would otherwise be left to decay or burned without energy recovery gives a clearer benefit when used for heat. A residue that would otherwise become a long-lived panel does not, because the energy route releases the carbon at once; the benefit then depends on the fossil fuel displaced.
2. Efficiency matters. Efficient use (a bark boiler replacing fossil kiln heat) differs from a low-efficiency power plant.
3. Forest residues are a different case. Extracting logging residues can lower forest carbon stocks and must be counted in the bioenergy system. This concerns logging residues, not sawmill by-products.

No GHG factor (kg CO2e per tonne or per MWh) for residue combustion was verified in the portal's sources, so none is given.

Checklist: a claim about residue climate benefit

• What happens to the residue otherwise (decay, panel, landfill, uncontrolled burning)?
• What fossil fuel does it displace, and with what efficiency?
• Is it a mill by-product or a forest residue?

Key points

• Carbon is about 50% of oven-dry mass; CO2 = C x 44/12. Reproduce declared biogenic values as a check.
• Default half-lives (Swiss inventory, IPCC 2014): sawnwood 35 years, panels 25 years, paper 2 years.
• Substitution factors (about 0.80 tC/tC for sawnwood) are averages that should fall as alternatives decarbonise; claim them only through module D.
• IEA Bioenergy: efficient use of sawmill by-products is typically linked to short-term climate benefit; the counterfactual decides.
• Forest-residue extraction is a different case from sawmill by-products.