In my team we are, just like the rest of the industry, starting to develop more and more timber schemes as an attempt at reducing the carbon emissions associated with our designs. One of our strongest arguments for selecting mass timber is formed based on LCA calculations of embodied carbon, and comparisons to other materials, where timber often (but not always!) performs favorably. We have been using the carbon counting tool developed by EWLP and the IStructE, along with the SCORS carbon rating system for structures.
However, this tendency towards timber means we need to be very clear in our understanding of exactly what it is we are calculating and what assumptions we are making about timber, so that we can communicate these clearly and we are able to provide actual science-based best options, and not just trendy timber buildings.
Of course, embodied energy is only half the picture (or more accurately, 2/3rds), and the choice of materials for a building will affect the design in far more ways than just the embodied carbon, but today we are going to simplify and stick with embodied carbon only.
Let’s start with this graph
Here we see an example of the embodied emissions associated with the life of a timber building or product. First, a large upfront spike resulting from harvesting, transportation, processing, and construction. In the LCA terminology, this is known as the stages A1-A5. Then follows stage B, the life of the building, where emissions due to operation, maintenance and refurbishment lie. As we are considering only the embodied carbon, these have no impact (repairs/replacements ignored for now), however, the graph is reducing over the lifespan. This is due to the sequestration or storage effects of timber on CO2. Finally, stage C, the end-of-life, and stage D, possible benefits of future reuse or recycling. The A1-A5 stages are relatively uncomplicated and are generally well grasped by most, so I’ll leave them out for the remainder of the post. Instead, we’ll discuss stages B, C and D in turn.
Timber sequestration and carbon storage:
The negative gradient in stage B comes from the fact that trees, like all plants, absorb CO2 as they grow. The assumption is that you are harvesting your timber from a sustainably managed forest, where the rate of planting matches the rate of felling, so that, when a tree is cut down to become available as timber in your building, which happens on a 20-40yr cycle, depending on the species, it is immediately replaced by a new seedling which will begin to grow and absorb CO2.
Then there are two main ways of thinking about this effect in our calculations: backwards- and forwards-accounting, or more simply storage or sequestration. It is important that you count only one of these effects, as they are essentially two sides of the same coin, but both are valid and are recommended by various standards and institutions.
The backwards-accounting (BWA) approach considers time = 0 to be the moment the tree, that you select for your building, began to grow, i.e. 20-40 years in the past. Then you consider the carbon stored in that specific tree to have a negative emissions value (as it is no longer in the atmosphere) in your calculation. You consider that tree only, and not any replacement trees. At end-of-life, the carbon stored is (potentially) released back into the atmosphere.
Forwards-accounting (FWA) considers time = 0 to be the moment you decide to cut down a tree, whereupon the new tree that replaces yours will begin to absorb carbon from the atmosphere. Here the actual carbon removed from the atmosphere by that replacement tree provides the negative emissions effect that shows up in our calculation, and at end-of-life this effect is countered by the carbon stored in the timber in the structure.
In BWA, you are taking credit for something that occurred in the past, which was entirely unrelated to you or your building. But you are also only considering the actual timber in your building, which is sensible. This method is recommended by the European norm on the subject.
In FWA, the opposite is true, but you are also considering the specific effect of your decision to use timber in your project (It’s a little funky, because you stopped that specific tree from absorbing more carbon, but allowed a different tree to begin). You are measuring what the atmosphere actually ‘feels’ in terms of net CO2 balance, because of your material choice.
Practically, there is no difference between these methods, neither in the end results of our calculations, nor in the forest (the effect is the same – you cut down the same tree), so it mainly semantics. What is important is what happens at end-of-life, as this is where we really start to be able to reduce our carbon footprint.
But before we get to Stage C and D, I want to look at a question I had about timber sequestration and the graph shown above. To me it was unclear how the sequestration was calculated.
As mentioned earlier, a typical growth cycle for managed forests is 20-40yrs,depending on the species that is grown. Let’s say our building has a design life if 100 years. 100 years is potentially several times the life of a managed tree, but it would seem that we are counting sequestration for a significant portion of the design life of the building, if not the entire design life. This would mean we are counting the carbon absorbed by not only the replacement tree, but also the replacement for the replacement tree, and the replacement for that, down till the nth replacement. However, the effect of all these trees beyond the first replacement are conceivably counted in subsequent LCA analyses for the buildings using those replacement trees, so we would be counting double, triple, quadruple and so on. To avoid that, we must necessarily be counting only our direct replacement, but is that the case? It is not evident from the graph.
What the graph hides is that the way LCAs are typically calculated uses a static value of the carbon in a unit of timber, converted by molecular ratio to carbon dioxide. This means for a sum of 1000t of timber used, a fixed amount of carbon is calculated, 1000c, which is simply distributed over the design life in a fashion that mimics natural timber growth. This confirms that we are only considering the material directly used in our design, aligning with the BWA approach. What we are not doing is taking dynamic carbon absorption values for tree growth stages, i.e. we are not actually modelling any real timber growth. This is simpler and probably best for our purposes, as an actual forest carbon balance simulation must include soil and ground cover and is therefore is extremely complex. Some researchers have presented dynamic LCA models, which do exactly, this, but these appear to be targeted as larger scale LCAs of whole forest systems, rather than buildings.
Stage C
Stage C represents end-of-life, which involves demolition and deconstruction, but is often dominated by how the material is disposed of, especially in the case of timber. Generally, four options are considered: landfill, incineration, recycling, and reuse. The carbon impact of the four options lies in that same order, where landfill is associated with high methane emissions, incineration is a simple return to atmosphere of the stored CO2, and recycling and reuse both extend the storage period of the carbon in the timber. It is important to note that, in the end, the stored CO2 will return to the atmosphere. There is no permanent removal of CO2 from the atmosphere in biogenic carbon, not in theory at least. However, timber and forest cover remains an extremely effective way of pulling CO2 out of the atmosphere and the duration of the storage will match the duration of the forest ecosystem, providing a strong argument for the development of additional high biomass forest land. If these ecosystems are allowed to exist indefinitely, the storage period will also be indefinite.
It is here we can see how the cascading use of timber could help us store significant amounts of carbon dioxide for a significant time period, extending our window of opportunity for mitigating the worst effects of climate change, while also providing us with a material to construct our houses and furniture out of. Naturally, not building at all and letting the forests reach maturity would store more carbon, but this is unlikely to be an acceptable path forward. So unlikely that it becomes more of a strawman than an actual argument.
In the case of 100% incineration, the CO2 emitted at end-of-life is equal to that stored in the timber, and data shows that downcycling of timber products (typically into chipping, mulch or fibres) has a similar processing cost in terms of emissions. It therefore becomes clear that timber cannot, on a whole-life basis, be carbon negative. A single project could be carbon negative, if the timber is reused without significant reprocessing, but this merely shifts the re-release of stored carbon outside the current LCA scope boundaries. In the end, the carbon will return to the atmosphere. While Bioenergy with Carbon Capture and Storage (BECCS) technology is being developed, we are a while away from implementation, and I prefer not to consider BECCS too seriously, as it would allow us to do very little in the way of actually changing the way we build, similar to wider arguments around climate change mitigation via behavior change versus technological innovation. Better to have BECCS as a surprise ace up our sleeve.
Stage D
Stage D encompasses benefits of future reuse and various additional effects which impact carbon and have no other natural place to sit. For timber, an additional ‘negative’ is included here, if stage C includes incineration. This negative comes from the energy produced during incineration and is calculated as the relative difference in the emissions for an equivalent magnitude of energy produced using fossil fuels versus timber.
When this is included in the calculation, it benefits the carbon accounts of a specific project to incinerate the timber at EOL, which is misleading, and potentially counter argumentative, if you are trying to make a case for reuse and recycling. This is why it is important to understand what is going on, and to provide evidence of how alternate reuse can provide even greater benefits. Additionally, as the future energy grid is decarbonized, the benefit of incineration will reduce and eventually disappear, so for new buildings that expect to last 60+ years, the point may already be moot, especially considering current global timelines for reductions in emissions and a decarbonized grid.
Summary
The SCORS tool:
