Carbon cycle & biogenic carbon

Photosynthesis fixes atmospheric CO₂ into organic matter; respiration, decay and combustion return it. The fluxes are large — terrestrial photosynthesis takes up carbon on the order of a hundred-odd gigatonnes a year, with a comparable return — against an atmospheric stock that is itself a few hundred gigatonnes larger than it was before industrialisation. Two features of that picture do the work for the bioeconomy: the biological cycle is fast, and it is close to balanced.

Biogenic versus fossil carbon

Chemically, a carbon atom from a plant and a carbon atom from crude oil are the same atom. The distinction is about the reservoir and the timescale it was resident in. Fossil carbon has been out of the atmosphere for geological time; burning it is a one-way transfer that adds to the active carbon stock. Biogenic carbon was in the atmosphere recently, was fixed by a living organism, and would have returned to the atmosphere on a timescale of years to centuries regardless. Releasing it returns carbon that was already circulating.

That is the entire physical basis of the “bio-based is better” argument, and it is a real one. It is also narrower than it is usually made to sound.

Where the neutrality assumption breaks

Timing. Carbon neutrality over a full rotation is not neutrality now. A forest burned today and regrown over sixty years releases its carbon immediately and recovers it slowly — a carbon debt with a payback period. Whether that matters depends on whether the target is cumulative emissions or a near-term peak.

The counterfactual. The right question is never “was this carbon biogenic?” but “what would have happened otherwise?” Biomass diverted from a use where it would have been sequestered, or grown on land that was storing carbon as forest or peat, carries that loss. Land-use change, including indirect displacement of production elsewhere, is where most of the contested emissions in bioenergy accounting sit.

Permanence. Carbon held in a product is only removed for as long as the product lasts. A structural timber holds it for decades; a fuel holds it for minutes. Soil carbon is reversible under a change of management. Durability is a property of the destination, not of the origin.

The rest of the balance sheet. Cultivation, harvest, transport and conversion have their own emissions, often fossil, and nitrogen fertiliser releases nitrous oxide — a far more potent greenhouse gas than CO₂. A bio-based product can carry a larger footprint than the fossil one it displaces.

How it is handled in practice

Life-cycle assessment conventions typically account biogenic uptake and release separately from fossil emissions, so that biogenic carbon nets out over the assessed period unless something prevents it — land-use change, delayed regrowth, or storage in a durable product, which is credited only where the storage is real and its duration stated. Different standards make different choices here, and the choice is not cosmetic: it can decide whether a product appears beneficial.

The practical conclusion: “biogenic” describes where a carbon atom came from. It does not by itself say what its release does to the climate, and only a full accounting — timing, counterfactual, permanence and the fossil inputs alongside — answers that.