Sequestration of CO2 in wood and other biobased products (carbon uptake) is a confusing subject in LCA:
The short term carbon cycle versus the long term carbon cycle
Part of the confusion is related to the difference in the so called short term carbon cycle and the long term carbon cycle. The long-term carbon cycle operates over geological timescales, spanning millions of years, and includes the reserves of coal, oil and gas. The short-term carbon cycle operates over much shorter timescales, ranging from days to centuries. The carbon of the short term cycle is called biogenic carbon.
Burning fossil fuels bring carbon from the crust of the earth into our atmosphere, resulting in global warming. It is regarded as a “linear system”: the end-of-life of the CO2 is in the air (although there are carbon sinks of organic matter the ocean that may end in the long term cycle)
Burning biobased carbon brings also carbon to the atmosphere, however this carbon originates from plants that captured the same amount of carbon, so there is a fundamental difference in source and a difference in timespan. It is regarded as a “circular system” in the biosphere. The assumption is that at the end-of-life of the product, the carbon goes back to the atmosphere, either by burning, or by any aerobic bacteriologic process.
The issue of circularity of biogenic carbon
There are three issues in LCA that cause confusion:
Issue (1) the loss of biodiversity in forests, versus the issue of carbon sequestration in forests
Issue (2) the requirements of circularity in the biosphere of biogenic carbon
Issue (3) the time span estimate of the product, i.e. how long does the “biogenic carbon uptake” lasts
Issue (1) The issue of biodiversity is blurred at internet discussions with the issue of carbon sequestration in forests. Keeping forest “untouched” is extreme beneficial for biodiversity, however, anaerobic bacteria might emit methane, which is quite negative in terms of greenhouse gasses (1 kg CH4 is 30 kg CO2 equivalent). This requires well balanced decisions: it does make sense to give biodiversity the priority in forests where biodiversity is high (i.e. topical rain forests), and give carbon sequestration the priority where biodiversity is low (i.e. boreal forests).
Degradation of biodiversity is calculated in eco-costs via land-use change (LUC), see the webpage on this issue at this website
Issue (2) requires to keep 3 different systems apart in the discussions:
(a) In tropical forests: modern rotational harvesting,
(b) In tropical forests: traditional clear cutting.
(c) Boreal forests in the Scandinavian countries: continuous clear cutting of small areas plus replanting.
Only FSC wood guarantees more or less method (a), which is a prerequisite for the assumption of circularity. Other than FSC wood originates unfortunately from method (b), often to create agricultural land, which is far from the circularity requirement. Note: the negative effect of method (b) is handled in the eco-costs system by (i) the land-use change, based on local loss of biodiversity (ii) the average loss of carbon sequestration of 3.45 kg CO2/ kg wood
The boreal forest in in Scandinavian countries, method (c), are continuously increasing their stored carbon in forests. The circularity of this wood is therefore guaranteed (there are more trees planted than harvested). It is a widespread misunderstanding that the period of regrowth of trees must be taken into account: from systems point of view, there is a condition of steady state i.e. for one specific area there might be a specific age of trees, but overall the ages of the trees are equally spread over the country, and that situation does not change much over time.
Issue (3) An arbitrary choice has been made in the LCA community: when the life span of a product is shorter than 100 years, the biogenic carbon belongs to the short cycle, and is therefore regarded as circular (the choice of 100 years is related to the predominant choice of the 100 years GWP midpoint tables). In this way complex calculation on dynamic systems, and the theory on the ‘delayed pulse’ are avoided. PEF follows this simplification as well, but proposes 300 years, instead of 100 years, as cut-off point. The eco-costs / Idemat system proposes 100 years (i.e. wooden beams in buildings are assumed to be there ‘for ever’): in such a case 1.72 kg CO2 per kg dry wood, or 1.51 kg CO2 per kg wood MC 12% should be added for carbon sequestration.
Note: a detailed analysis on the mass balance for wood and bamboo is provided in (Vogtlander, et al 2014). See also the webpage Eco-costs calculations on wood
Two ways to deal with the circularity
In LCA there are two ways to calculate the biogenic CO2 cycle:
(A) Do not count biogenic carbon (e.g. biogenic CO2 = 0), since the short life biogenic CO2 is circular (the biogenic CO2 stays within the boundary limit of your calculation system, and all uptake of biogenic carbon will be released at the end-of-life sooner or later).
(B) Carbon uptake at the moment of harvesting is counted, combined with release of the carbon at the end-of-life that is counted as well. This is the case where the atmosphere is not included in the product system (the biogenic carbon enters the system at the cradle, and leaves at the end-of-life)
The effect on your calculation is depicted in the 3 Figures below (from CEFIC position paper 2022):
The first figure depicts the situation for CO2 from fossil fuels. The second figure depicts calculation (A) for biogenic carbon. The third figure depicts calculation (B), which shows a negative carbon score for e.g. wood or bioplastics.
It is clear that companies like to have a negative carbon score for cradle-to-gate in their marketing (“we are a carbon negative company”). Scientist, however, regard this as greenwashing (misleading consumers) since the score for the total LCA (cradle-to-grave) is positive. So far, PEF rejected the calculation (B).
Some history on the issue.
In November 2009 It was decided by Ecoinvent to follow the IPCC in their decision in 2006 to apply the calculation system (A) “biogenic CO2 is not counted in LCA”
In the ILCD manual on LCA of 2010, the same approach is followed.
In the EN15804 (the norm for EPDs) allows calculation method (A); carbon uptake may be reported just as as additional information, however, not as an integral part of the LCA calculation.
PEF did follow the EN 15804 approach, for products with a life span shorter than 300 years
IDEMAT follows calculation method (A). The carbon uptake might be reported separately from the LCA calculation (for wood the uptake is 1.53 – 1.83 kg CO2 per kg dry wood) for products that have a shorter life span than 100 years. Carbon sequestration must be included in LCA for products that last longer then 100 years
Consequences for combustion with heat recovery.
The consequences for the end-of-life calculations on “combustion with heat recovery” is that the end-of-life scores for wood and biobased products become negative. See FAQ 2.4. This is the basic idea of “avoided fossil fuels”: at the end-of-life heat is recycled with a positive effect (a ‘credit’) on the environment (compared to doing nothing).