Biogenic carbon in timber buildings – how should it be considered?

Methods of carbon accounting have, in some circumstances, considered the ongoing sequestration of the replanted tree as a means to draw down the embodied carbon of a building that uses timber¹. It could be argued that this is no different than a form of carbon offsetting that could be adopted by any form of construction. Although harvesting trees can enable greater levels of net carbon storage when well managed (see Figure 2), any ongoing sequestration in the forest ecosystem should be considered separately to the cumulative carbon emissions of a given building as illustrated in Figure 1.

Would it be better if trees were just left in the forest?

Forests are often described as ‘carbon sinks’, and the impact of harvesting trees for construction or other uses must ensure that it does not hinder the ongoing sequestration. It is generally understood that sequestration rates during the life of a tree follow an ‘S’ curve: initially slow, then accelerating before eventually reaching a steady state.

For a managed forest, if you amass the stored carbon in the products produced from the harvested trees with the carbon that continues to be stored by the trees planted in replacement, the total sequestered carbon could eventually be greater than that of an unmanaged forest, as shown in Figure 2. This is not to say that carbon stored within a forest should be reported as a benefit to the LCA of a building; this is more to justify the harvesting of a tree for long-term carbon storage in the form of a construction product. It is also dependent on the context: if the tree is not replaced (deforestation) then future sequestration should not be assumed, and the use of timber is clearly detrimental, although this is uncommon in the EU.

Industry guidance and Ramboll’s design stance

The IStructE guide², ‘How to calculate embodied carbon’, advises that, when considering a whole life carbon assessment, the ‘store and release’ approach must be adopted, meaning that when biogenic carbon is considered as a negative value at the construction stage of a project, then it must be assumed to be released at the end of a building’s life. It is also noted that, based on current practice, '86% of sequestered carbon (is) lost within a century'³, which although is likely to be beyond the design life of the structure, is a reminder that stored biogenic carbon cannot be assumed as a permanent reduction of embodied carbon.

'The conservative approach to assessing biogenic carbon in building LCAs is to assume that the biogenic carbon is stored only temporarily, reverting to the atmosphere within the timeframe of the assessment, and therefore has no overall impact.'⁴

The Ramboll design stance, rather than temporarily considering the storage of carbon, is to separate stored carbon from building carbon assessments, but to acknowledge stored carbon when reporting whole life cycle assessments. This means not conflating any biogenic carbon with the fossil carbon in a carbon assessment at any point in the lifecycle.

However, we advocate that the stored biogenic carbon in timber, and other biogenic materials, should be recognised and recorded. A designer, in discussion with relevant stakeholders, can then agree what approach should be assumed depending on the certainty of the end-of-life scenario, confidence in the ability to extract timber for future re-use, and assumed timescales of the reporting. This can unlock discussions on how to advocate for maintaining stored carbon in future buildings or uses.

Custodians of carbon: A call to action

All those in the built environment have roles and responsibilities to play to help increase the global storage of carbon in an attempt to reduce the effects of climate change.