Is carbon sequestration in timber buildings real?

Toby Maclean, posted 17th December 2020

In recent years and particularly in recent months I have heard more and more discussion about 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” is basically a slightly shorter and slightly posher way of saying “to store safely”. The idea is that the CO₂ that 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 for the building. Just on a technicality, it is carbon that is locked away in trees, not CO₂. It’s is just that the carbon in trees has the potential to become CO₂ in the atmosphere.

It 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.

At the other end of the spectrum I hear that CO₂ sequestration in buildings is lot of nonsense and we may as well use concrete for our log cabins.

The former idea, that using more and more timber in buildings will reduce atmospheric CO₂, is of course 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 then the forestry is not sustainable. And sustainable forests are a finite resource.

But 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.

The latter idea, that CO₂ sequestration in buildings is some kind of green smokescreen, is (probably) based on trying to work out how the whole sequestration gig works but missing the bigger picture. For example, if you consider a tree that takes 50 years to grow before it is felled and then you consider that only some of that tree goes into the building and the rest goes into shorter lifespan products (there’s a long list of things that could be added here but at the extreme some of the alternate products may have a lifespan of zero years). The fate of the thinner branches and roots may also cause some concern and 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 end of the building life as if it is burnt or decays then all the CO₂ that you thought was being stored is all released anyway. And if you then consider how long it took the tree to grow before it was felled you may also start to feel that there is quite a long payback in recouping the CO₂. And before you know it, if you are the kind of person that designs buildings, you may be reaching for your reinforced concrete design manual.

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

Here are those reasons:

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 (unless perhaps you are specialist toothpick manufacturer). If you try and do this then you will find you are sitting on your hands for an awfully long period after each felling waiting of the forest/clump of trees to grow again. But if you have a forest, or collection of forests, that is large enough to fell annually on a 30 or 40 or 60 year rotation (or whatever the optimum rotation may happen to be for your forests) then you never have to sit on your hands, you’ll be too busy chopping trees down continuously, perhaps while whistling a well known ditty made famous by Monty Python’s Flying Circus. And 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.

Figure 1 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 shows a collection of forests with CO₂ being captured at the same rate it is lost; the minimum required for it to be climate change neutral. Figure 3 shows the result of a climate change neutral forest on the Figure 1 style graph (with the sub-annual fluctuations averaged out).

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 of course the carbon emissions from your vehicles and chainsaws and checked flannel shirts offset this.

Figure 1

Figure 2

Figure 3

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 and to 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 (ie: you fell the same amount of trees each year) then 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 (ie: 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 then you double the total carbon store you can achieve.

Figure 4

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 but as a minimum, even if burnt at zero years it is more or less carbon neutral and if it is burnt at zero years as an alternative to burning a fossil fuel then there is a net benefit.
And what if you had not felled the trees at all but just left those forests to grow? This is probably a bit more complex. On one hand older, especially 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) [source] while on the other a well established tree can absorb more CO₂ per year than a number of younger trees combined [source]. But actually 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? Because 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 then we will reach a steady state in the CO₂ stored in timber buildings and 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 in timber? 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 into the bargain. What it does mean is that we should:

Ensure buildings (whether timber or not) last forever, or if we can’t quite manage that then for a very long time indeed.
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.

And as a footnote, here are all the other good things about doing the above that are not mentioned already:

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. And if that were easy we’d have done it by now
Avoiding more carbon-intense buildings. All other things being equal then not only may a timber building help increase or maintain the CO₂ sequestration level but it is also inherently fairly low carbon (ignoring the sequestration effects) compared to many other building materials and 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) estimate 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 (forestry activitiesd can release it as well as add to it) and it is important that forestry activities do not disturb that balance.

But none of this is an excuse to ignore the first rule of climate change club which 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.

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