22 January 2021

Is carbon sequestration in timber buildings real?

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Article written by Toby Maclean of Allt environmental structural engineers

 

As more people become aware of the impending climate crisis, discussion has increased around carbon dioxide (CO2) sequestration in timber products. CO2 sequestration in timber is most often thought about in terms of how timber-based buildings will sequester CO2 that was locked in the wood by photosynthesis when the tree was growing. The word “sequester” basically means “to store safely”.

 

The idea is that the CO2 the tree absorbed in order to grow bigger and taller is now locked away in the building, while other trees have taken the place of the ones that were felled. (In fact, it is carbon that is locked away in trees, not CO2. However, the carbon in trees has the potential to become CO2 in the atmosphere.)

 

This leads to the alluring idea that we have a CO2-negative building material – the more of which is used, the less CO2 is in the atmosphere. That is, it is possible to actually reduce CO2 in the atmosphere by using timber in buildings. This would obviously be a good thing as CO2 is the most significant greenhouse gas, and greenhouse gases cause the climate change that we worry about.

 

Of course, the idea that using more timber in buildings will reduce atmospheric CO2 is limited by the fact that the whole idea of CO2 sequestration is underpinned on a premise of sustainable forestry. If the timber in the building is not being replaced in the forest at the same rate that the timber is put into buildings, the forestry is not sustainable. And sustainable forests are a finite resource.

 

The supply of sustainable timber is not the only thing that limits how much CO2 ends up stored in buildings – the lifetime of the timber in the building is also an equally significant factor.

 

Some people argue that CO2 sequestration in buildings is a kind of green smokescreen. This is probably due to trying to work out how the whole sequestration gig works but missing the bigger picture. For example, consider a tree that takes 50 years to grow before it is felled; only some of that tree goes into the building while the rest goes into shorter lifespan products.

 

There’s a considerable amount to be discussed here but, at the extreme end, some of the alternate products may have a lifespan of zero years. The fate of thinner branches and roots may also cause some concern; one may be tempted to offset the CO2 release for the “rest of the tree” against timber in the building. One may also be concerned about the fate of the timber in the building at the end of the building's life, because all the CO2 that you thought was being stored is all released anyway if the building is burnt or decays.

 

Considering how long it took the tree to grow before it was felled, you may start to feel that there is quite a long payback in recouping the CO2. Before you know it, you may be reaching for your reinforced concrete design manual.

 

The good news for supporters of CO2 sequestration is that it is real, and the concerns listed above are generally unfounded. But the bad news is that the reasons the concerns are generally unfounded are also the reasons that the level of CO2 sequestration cannot grow indefinitely.

 

You can’t think of a sustainable patch of forestry in isolation.

A patch of forestry is only sustainable if it is large enough to sustain commercial rotations of felling forever. A single tree can never be grown and felled sustainably and neither can a small cluster of trees. If you try and do this, you will find you are sitting on your hands for an awfully long period after each felling, waiting for the forest or clump of trees to grow again. However, if you have a forest (or a collection of forests) that is large enough to fell annually on a 30, 40 or 60 year rotation (or whatever the optimum rotation may happen to be for your forests), you never have to sit on your hands: you’ll be too busy chopping trees down continuously. The fact that you are never waiting for trees to grow means you are also never waiting for carbon to accumulate in your forests. You are, at the very least, matching the carbon increment each year in your forests to the carbon decrement each year from all the trees you chop down.

 

In practice, you will probably have a larger carbon increment than carbon decrement and so will be increasing the carbon store in your forests year on year, although the carbon emissions from your vehicles, chainsaws and checked flannel shirts offset this.

Figure 1: CO2 store in a stand of forestry. This shows the “wait for your trees” model which implies your forest is not big enough; the tip of each sawtooth is the year in which your entire forest is felled.

Figure 2: The annual carbon increment over sustainable forests is at least equal to the annual carbon decrement and so the total carbon store in the forest biomass remains constant or increases. This shows a collection of forests with CObeing captured at the same rate it is lost; the minimum required for it to be climate change neutral.

 

Figure 3: CO2 store in sustainable forestry biomass. This graph shows the result of a climate change neutral forest on the Figure 1 style graph (with the sub-annual fluctuations averaged out). 

 

You can’t think of a building in isolation.

For CO2 sequestration in buildings to work, you need to build lots of buildings in timber, continue building them and preferably never knock them down. The expected lifetime of buildings is a whole other topic. But, just like the forests, the idea is that as you build up your pool of stored CO2 in buildings then as buildings are torn down and burnt (for reasons that currently elude me), the rate of CO2 sequestration equals the rate of CO2 release and you have a CO2 store that neither grows nor shrinks over time.

 

Figure 4 shows the carbon store that can be created in timber products –not just in buildings, although buildings tend to last longer than most other timber products. The carbon store grows until the rate at which it is losing carbon equals the rate at which it is fed and then enters a steady state (the flat line bit of the graph) for as long as it continues to lose and gain carbon at the same rate. If the rate at which the carbon store is fed remains constant (i.e. you fell the same amount of trees each year), the start of the steady state point is whatever the maximum life of any timber product is before the carbon is released from that product. If you double the average life of the timber products, you will double maximum carbon you can store. By the way, we're not at year zero right now – we are already some way up the sloping bit of the graph.

 

If you also increase the amount of carbon you feed into the store over time, i.e. you increase sustainable forestry activities, you will also increase the total carbon store. All other things being equal, if you double the timber you turn into products, you double the total carbon store you can achieve.

Figure 4: Ratio of overall size of carbon store in timber products to carbon in timber felled annually.

 

What about all the timber felled from the forests that didn’t make its way into a building?

Well, that’s regrettable from a CO2 storage point of view if that timber had a shorter life as a result. However, as a minimum – even if burnt at zero years – it is more or less carbon neutral. If it is burnt at zero years as an alternative to burning a fossil fuel, there is a net benefit.

 

What if you had left those forests to grow?

This is probably a bit more complex. Older, often unmanaged forests start to slow their CO2 capture rates. (Canadians are probably rather surprised to learn their forests are currently carbon positive, emitting more than they absorb.) In contrast, a well-established tree can absorb more CO2 per year than a number of younger trees combined. But truly sustainable commercial forestry does not happen by accident; if there were no demand, those trees would not exist (although others might) and different forest management strategies also have a large influence on the CO2 absorption rates of forests. There may well be a certain use of land that is more effective at absorbing CO2 than commercial forestry and it should be encouraged, but it will not produce timber for buildings.

 

So why should all these good reasons temper the evangelistic leanings of fans of CO2 sequestration buildings? It means that if we maintain timber building rates and if we allow those buildings to be burnt or to decay at some point in the future, we will reach a steady state in the CO2 stored in timber buildings. All future building is doing is offsetting the end-of-life release of CO2 from older timber buildings.

 

Does that mean we should not use buildings to sequester CO2?

Of course not. Growing trees in the right places and storing their timber in buildings, while it may not be the absolute optimum tree-based carbon storage scheme, does comes with the significant benefit – amongst others – that you also get buildings in the bargain. This means that we should:

  • Ensure buildings (whether timber or not) last forever, or – if we can’t manage that –for a very long time.
  • Maximise the timber production from the existing commercial sustainable forestry.
  • Increase the area of land devoted to commercial sustainable forestry, balanced against other important uses and diversity considerations.

 

Final points

CO2 released now is worse than CO2 released in the future. There is not actually much point in getting to the future unless we do a good enough job at controlling CO2 emissions now. Timber buildings are also inherently fairly low carbon (ignoring the sequestration effects) compared to many other building materials, so there is usually a net benefit just from substituting a non-timber building with a timber building.

 

Soil carbon

When forests grow, they accumulate carbon in the trees above ground and roots below ground, but the soil also accumulates carbon. Forest Research (part of the Forestry Commission) estimates that the top 1m of soil in forests in the UK contains more than double the carbon in all the woody biomass (dead or alive) in the same forests. Soil carbon may be a delicate balance, however, since forestry activities can release it as well as add to it. It is important that forestry activities do not disturb that balance.

 

CO2 reduction

The first rule of fighting climate change, however, is to reduce all CO2 – and other greenhouse gas – emissions as much as possible by consuming less (burgers, TVs, and buildings). The only hope for carbon capture of any kind is if emissions have already been drastically reduced.

 

About the author

Toby Maclean has many years’ experience of engineering technically demanding projects of all kinds from bespoke furniture to cultural institutions. His approach to design is from first principles, which ensures his skills can be adapted to all fields without limitation by precedent. In 2020, he established Allt environmental structural engineers in order to address the urgent need to decarbonise the built environment with a particular emphasis on carbon embodied in structures.