Circular wood use can accelerate global decarbonisation but requires cross-sectoral coordination

Predominantly linear use of wood curtails the potential climate-change mitigation contribution of forestry value-chains. Using lifecycle assessment, we show that more cascading and especially circular uses of wood can provide immediate and sustained mitigation by reducing demand for virgin wood, which increases forest carbon sequestration and storage, and benefits from substitution for fossil-fuel derived products, reducing net greenhouse gas emissions. By United Kingdom example, the circular approach of recycling medium-density fibreboard delivers 75% more cumulative climate-change mitigation by 2050, compared with business-as-usual. Early mitigation achieved by circular and cascading wood use complements lagged mitigation achieved by afforestation; and in combination these measures could cumulatively mitigate 258.8 million tonnes CO2e by 2050. Despite the clear benefits of implementing circular economy principles, we identify many functional barriers impeding the structural reorganisation needed for such complex system change, and propose enablers to transform the forestry value-chain into an effective societal change system and lead to coherent action.


Circular use:
To match the research objectives of the paper, we selected recycled MDF as the circular case study because it is an almost closed-loop process (hence it is a circular, rather than cascading, use).It was also determined to be a suitable scenario for this study because it applies proven technology to produce (a recycled version of) an already established product with a known market size in the UK; and the collection of waste wood (including MDF) is already common practice in the UK.Furthermore, we were able to obtain sufficient data to meet the requirements for rigorous LCA, specifically: 1) the annual volume of recovered MDF at the UK national scale of our study 2) the volume of MDF currently manufactured in the UK 3) lifecycle inventory data for the process (which uses proven technology that is being commercially scaled-up)

Cascading use:
The UK value chain also presents opportunities for improved hierarchical cascading use.The major feature of the cascading wood use case selected for the present study is enhanced hierarchical use, which inherently leads to a higher degree of cascading use (i.e.sequential use of wood in multiple applications).In the UK there is very little landfilling of waste wood, and the majority of recovered waste wood goes into cascading uses: primarily particle board manufacturing (which has 50% recycled wood content in UK) and bioenergy.By increasing the volume of sawnwood produced, the future volume of recovered waste wood (sawnwood) cascading to other uses (in our example particle board manufacturing) also increases.
There are significant additional opportunities for enhancing and adding further levels of cascading use to the UK wood value chain.However, our selection of pathways and scenarios for analysis was governed by the stringent data requirements of the rigorous LCA methodology that we have utilised.
We carefully surveyed a range of other existing pathways and scenarios but due to their complexity and major gaps in available data, none of them met the criteria of being suitable for rigorous LCA.This limitation was particularly acute for emerging technologies.This represents a conservative approach, given that various prospective sequential uses (examples below) could further increase the extent of, and GWP credit derived from, cascading and/or circular value chains in future.

Alternative wood use scenarios
There are many other promising cascading and circular economy value chain alternatives, at different stages of development and application that could be derived from wood.These include:  Bioplastics/biocomposites -these can replace many types of petrochemical plastics and can be recyclable  Biobased chemicals -these can have multiple applications (with varying product substitution impacts), including: biobased fragrances and flavours; lubricants; resins; cleaners and solvents  Biobased materials, such as microfibrillated cellulose (MFC), carbon fibre and bindersthese have a wide range of potential applications including packaging, textiles, energy storage (batteries) and construction products  Advanced second generation biofuels -biofuels manufactured from wood can replace fossil fuels 7 .
 Biochar -there is a small established market for biochar produced from pyrolysis of wood or other biomass, which can be used as a soil improver 8 and for many other purposes 9 .Application of biochar to the certain soils can store the contained carbon and keep it out of the atmosphere for many decades.Many more emerging applications of biochar are being developed, including addition to concrete to create a long-term carbon sink 10 .
 Mass timber, based on engineered wood products such as cross-laminated timber (CLT), dowel-laminated timber (DLT) or glue-laminated timber (glulam or GLT), manufactured from virgin wood, comprising multiple layers of solid wood -these are used to replace concrete and steel or conventional sawnwood beams in construction.
Production of these materials overwhelmingly uses virgin wood rather than recovered waste wood as the primary feedstock, at present.It is possible that in the future new processes will be developed enabling the utilisation of recovered waste wood.A specific promising example is cross-laminated secondary timber (CLST), which utilises recovered waste wood (e.g.recovered floorboards), and is a good example of cascading wood use.However, such a value chain remains uncertain and challenging to implement (and model) as technical barriers remain to be overcome, such as definition of the minimum level of quality and size of recovered sawnwood (strength grading is more complex with reclaimed timber).Even with more certainty on the specification of sawnwood eligible for CLST production, the collection and reporting of data on the quantity and quality of recovered sawnwood in the UK is insufficient to derive robust national scenarios -particularly where consequential GWP impacts of any changes would require information on how that wood is currently being cascaded.
In essence, there is a huge range of options for circular and cascading wood use that should be included in more prospective assessment of wood value chains in order to understand the full climate mitigation potential of wood.The examples mentioned above have limited availability (often due to commercial sensitivities) of the process data required to perform LCA (as listed in points 1 to 3 above).The products also have different qualities with respect to recoverability, recyclability and cascading use; and many will have considerably different product life durations, all with significant implications for lifecycle GWP impact.The scale of each of these product markets is also very uncertain, which will also have important implications on net demand and competition for (limited) resources, with complex consequential GWP impacts.Further study is urgently needed to assess these options as technical data emerges from pilot studies; in the meantime, the exclusion of these commercially unproven products is a conservative approach in this study with respect to our conclusions.

Supplementary Methods 2 -Decarbonisation Assumptions
Decarbonisation of the forestry value chain is highly dependent on the development of zero emissions mobile machinery and heavy goods vehicles (HGVs), as well as electricity and gas grid decarbonisation.A summary of the decarbonisation projections used in the study are summarised in Table 1 and described below.Calculations are also contained in 'Supplementary_Data_1_(BAU_LCA)'.

Forestry operations
Projections for forestry operations are taken from the Committee for Climate Change Sixth Carbon Budget 1 'balanced net zero pathway' for decarbonising heavy goods vehicles (HGVs) (optimistically, given the remote and arduous nature of operations); agricultural machinery has similar decarbonisation projections in the CCC report.

Sawmills & wood fuel production
Projections for decarbonising sawmills are calculated using the projections for forestry operations (mobile plant), the electricity grid and gas grid; and apportioned according to the breakdown of energy sources and tonnage (averaged across the sector) from primary UK sawmill data (2020) 14 .
Woodfuel production decarbonisation is assumed to be similar to sawmills since the process is also highly electrified and employs mobile plant for moving material around site.

Wood panel production
The Wood Panel Industry Federation (WPIF) commissioned a report 12 (by SLR Consulting Ltd) in 2022 to outline a decarbonisation pathway for the wood panel industry in the UK and Ireland.The

Table 1 -
Industrial decarbonisation projection assumptions for greenhouse gas (GHG) emissions from forestry operations, wood processing and transport, and from production of substituted products (concrete and fossil fuels).For clarity, 100% indicates zero GHG emissions.