Carbon footprint basics: LCA, EPD and GWP

Life cycle assessment in four choices

LCA (ISO 14040 and 14044) adds up environmental flows over a defined life cycle. Four choices define a sawmill result:

1. Declared unit, typically 1 m3 of sawn, dried, planed or rough timber at a stated moisture content and density.
2. System boundary. In the European standard EN 15804, cradle-to-gate covers modules A1 (raw material supply, including forestry), A2 (transport to the factory) and A3 (manufacturing: sawing, drying, planing, packaging). Later modules cover construction (A4-A5), use (B), end of life (C) and benefits beyond the system (D).
3. Allocation. A sawmill makes timber and co-products at once; impacts are shared by mass, economic value or system expansion. A US softwood inventory allocated by bone-dry mass, giving lumber about 50% of the sawmill-step impacts (50.1% in the Pacific Northwest). The method changes the result.
4. Data quality. One mill or an average, and which year? The Swedish Wood declaration uses 2019 data from 16 planing mills, about 25% of Swedish planed wood output.

EPDs

An Environmental Product Declaration (EPD) is a third-party-verified LCA result in a standard format. In Europe EN 15804 applies; the +A2 version (2019) splits climate impact into GWP-fossil, GWP-biogenic, GWP-luluc (land use change) and GWP-total. North American declarations follow ISO 21930. EPDs usually have a validity period of five years, so check dates.

Reported A1-A3 values, kg CO2 eq per m3

North American values use different methods and product definitions from the Swedish values; do not compare them line by line. The New Brunswick figure excludes drying and planing, so it is not comparable with the planed dry products above, and it excludes biogenic carbon.

Reading the numbers

• Fossil GWP tracks mill energy. Swedish values are about 29-49; North American cradle-to-gate values about 36-81. Grid mix, forestry diesel, transport and the fossil share of kiln heat explain much of the spread.
• A3 dominates. For the AWC/CWC declaration 42.56 / 63.12 = 67.4%, about two thirds. Manufacturing (electricity and drying) is the part the mill controls.
• Sums check. 10.55 + 10.01 + 42.56 = 63.12 and 7.59 + 3.99 + 34.28 = 45.86. New Brunswick: 13.8 + 9.8 + 17 = 40.6, which rounds to the reported 40.7; sawmill share 17 / 40.7 = 42%.
• Primary versus final energy. EPDs report primary energy; meters read final energy. The Swedish declaration gives 900 MJ non-renewable primary energy per m3 for a combination plant, about 250 kWh (900 / 3.6).

Worked example: GWP-total

GWP-total is the sum of fossil, biogenic and land use change terms. For the Swedish declaration, fossil 29.6 plus biogenic -773 gives -743.4, reported as -743. A Swedish sawn timber declaration from another producer lists 32.7 fossil and -800 biogenic: 32.7 - 800 = -767.3.

A strongly negative GWP-total does not mean the mill removed carbon from the atmosphere. It means the product contains carbon the tree absorbed; whether that is a climate benefit depends on forest regrowth and the product's fate, which a cradle-to-gate declaration does not cover.

Checklist: reading any footprint figure

• Which unit (m3 of logs, green, dried, planed) and which moisture?
• Which modules (A1 only, A1-A3, more)?
• Fossil only, with biogenic flows, or with land use change?
• Which year, region and grid? Which allocation method?
• Is the EPD still valid? Is it an average or a single mill?

Key points

• An LCA result depends on declared unit, boundary, allocation and data quality.
• EN 15804+A2 reports GWP-fossil, GWP-biogenic, GWP-luluc and GWP-total; GWP-total = sum of its parts.
• In the sources, A3 (manufacturing) is about two thirds to three quarters of cradle-to-gate fossil GWP in the two North American declarations.
• Fossil GWP is roughly 30-80 kg CO2 eq per m3 across the cited sources, with different methods; do not rank products by them.
• A negative biogenic figure is stored carbon, not proof of a climate benefit.