Timber as a Carbon Store
The reduction in global carbon emissions and achieving the impending 2030 and 2050 carbon targets won't be easy but the wider use of timber will help immeasurably, Callum Hill, Consultant at JCH Industrial Ecology Ltd explains why.
In September 2020 President of the European Commission Ursula von der Leyen in her State of the Union speech stated: "Our buildings generate 40% of our emissions. They need to become less wasteful, less expensive and more sustainable. And we know that the construction sector can even be turned from a carbon source into a carbon sink, if organic building materials like wood and smart technologies like AI are applied."
According to the World Green Building Council (WGBC), of this 40% of global carbon emissions, 28% comes from the operational carbon and 11% from the embodied carbon of the construction materials. In order to keep the global temperature increase below 2oC, all sectors of the economy must rapidly decarbonise. The vision of the WGBC is that by 2030 all new buildings, infrastructure and renovations will have at least 40% less embodied carbon and that by 2050 the embodied carbon should be net zero. These are extremely ambitious targets, so what does this mean and is it achievable?
The embodied carbon is the total of the greenhouse gas emissions that are associated with the extraction, processing, manufacture and transport of a product from the point of the raw resource in nature to the product leaving the factory gate. This is usually known as cradle to gate (modules A1 to A3 in EN 15804). Because different greenhouse gases have different impacts, the total emissions are converted into a common factor, known as the global warming potential (GWP), which is reported in carbon dioxide equivalents (e.g. kg CO2e). Sometimes, these are direct emissions – for example, the release of fossil carbon as CO2 during the conversion of limestone to clinker in cement manufacture. Quite often, these emissions are indirect, such as the release of carbon dioxide by power stations in the generation of electricity. Is it really possible to get these emissions down to zero? This is where the use of timber in construction can help.
Trees absorb atmospheric carbon dioxide as they grow and this carbon is stored in the wood (biogenic carbon). Eventually, the tree will die and the carbon will be returned to the atmosphere as the wood decays. In plantation forestry, this wood is harvested and can be used in harvested wood products (HWPs), which will store the atmospheric carbon for the lifetime of the product. Meanwhile, the harvested tree is replaced with new growth, so that the process of sequestration continues. The Intergovernmental Panel on Climate Change (IPCC) provides guidance on how to record carbon stocks in the different pools (forest, HWP). The principal is shown diagrammatically in Figure 1.
This shows the carbon cycle, with storage of atmospheric carbon in the forestry and built environment carbon pools with incineration at end of life and return of the carbon back to the atmosphere. The question is – if the carbon returns to the atmosphere at end of life, is there really any extra climate change mitigation benefit obtained from using timber in construction? This question can be answered by referring to the graph in Figure 2.
This shows a simple model where a constant amount of atmospheric carbon (stored in the timber) flows into the built environment carbon pool every year. The loss of this material is modelled over time, where all of this annual input is eventually lost. This loss is modelled for each year's input (this is called a distributed pool approach). The way this loss is modelled has been chosen to best represent what actually happens with buildings during their lifetime and represents all of the buildings in the built environment carbon pool. This is much better than considering a single building, where wildly different answers can be obtained, depending upon the assumptions made.
The effect of the lifetime of the timber products is illustrated by using a 50% loss time of 50 years and 100 years (similar to using the IPCC approach of half-lives). The result of this study shows very clearly the benefit of using timber in the built environment as a carbon store and the additional benefit gained by life extension of the timber products. This shows the benefit of using timber in the built environment as a carbon store, but what about the impacts associated with processing the material?
A recent report for the UK Climate Change Committee (UK CCC) showed that increasing the timber content of buildings produced a lower GWP impact (carbon footprint). The GWP of two building types used in the UK CCC study is shown below in Figure 3. The data is for a detached house and for medium-rise flats, either built using conventional masonry materials, or using high levels of timber (e.g. CLT in the flats). Here, the positive values (in black) represent the embodied carbon emissions and the negative values (in white) are the atmospheric carbon that is stored in the timber of the structure (even masonry structures contain timber, just less of it).
The increased use of timber in construction does not only store atmospheric carbon, but there is a measurable reduction in the carbon footprint (GWP) per m2 of floor area for an identical building. For the detached house, this saving amounted to 75 kg CO2 equivalents per m2 and for the high-rise flats 256 kg CO2 eq. per m2 floor area. Finally, at the end of life (or multiple lives, if the wood is cascaded down the value chain), the wood can be incinerated and the stored solar energy can be recovered and used to heat buildings or generate electricity.
The conclusion is that using timber to substitute for inorganic materials, such as steel and concrete, results in a lower level of emissions of greenhouse gases, plus there is an additional mitigation benefit to be gained, due to the storage of atmospheric carbon in long-life products in buildings. Extending the life of the timber used in buildings increases the benefit. The key is to design for a long life and ensure the potential for repurposing.